Monk Kokkalis and Grayson Pierce in James Rollins' Sigma Force series.
In Sherlock Holmes, the eponymous character and his "partner in crime" (who doubles as a chronicler of their adventures), Dr. John Watson.
Likewise, Hercule Poirot and his Watson, Captain Hastings. Poirot is unmarried, and generally avoids romance altogether. Hastings does eventually marry and move to Argentina with his wife, but remains Poirot's Sarcastic Devotee.
In Josepha Sherman's A Strange and Ancient Name, the main character (a half-elven (half-fairy?) prince) rescues Alliar, a wind spirit trapped in mortal flesh, and teaches the suicidal wind spirit to enjoy life in the flesh. They become very close friends, and the prince is about the only mortal Alliar can stand. When the prince's love interest almost writes him off - since he obviously loves Alliar and vice versa - Alliar explains to her that "flesh games" (sex) is so foreign to him, it's like trying to smell colors. And that's hardly the only place this is brought up. Kinda sad that our culture so identifies intimacy with sex that intimacy without sex must be made Anvilicious in order to even work.
Aziraphale and Crowley in Good Omens. Often Mistaken for Gay, even though they should be (im)mortal enemies as they're angel and demon, respectively. The other angels and demons are mostly busy trying to snare a soul here or there, or ignore creation completely, while Crowley and Aziraphale share a genuine interest in the world around them. They're just waiting for Armageddon. And humans, well, they're not around for as long, are they?
A lot of the confusion -- both in story and by fans -- comes from the fact that Aziraphale calls Crowley 'my dear' (he calls everybody 'my dear'), and Crowley calls Aziraphale 'angel' (he is one).
R & G even more so in the play Rosencrantz and Guildenstern Are Dead if only because they're the main characters in that.
Rosalind and Celia in As You Like It. Even marriage doesn't likely break it up, since they marry brothers.
Close friendships between men are a recurring theme in Tolkien's work; the reason possibly to be found in Tolkien's personal World War I experience, which shaped his belief that friendship was one of life's greatest gifts.
Legolas and Gimli turn into this over the course of the book, in the process overcoming Middle-Earth's long-lasting animosity between Dwarves and Elves. The appendices reveal that they took each other sightseeing through Middle-Earth, and finally sailed for Aman together on the last of the elven ships. In fact, Gimli is the only dwarf ever to make the journey to the Undying Lands, and he does it at least partially to be with Legolas.
Merry and Pippin are not seen without each other until they are separated by circumstance in The Two Towers, which is kinda a big deal for them (especially for Pippin, who learns in the process to be more responsible and less dependent on Merry).
Turin and Beleg in Tolkien's The Children of Hurin/The Silmarillion. They forego the comfortable Elven halls of Doriath to live together in isolation for years; then, when Turin is unjustly exiled, Beleg spends over a year searching for him in the wilderness, only to be captured and tortured to the point of death, rendering it nesesscary for Turin to swoop in and rescue him; and that's just the beginning. Indeed, due to women being fairly marginalized throughout most of the volume, Tolkien rather unsubtly uses Beleg as the Distressed Damsel of the narrative...the amount of times he is captured, imprisoned, and rescued at the last moment would make any number of female Mary-Sues jealous.
Kethry and Tarma, of Mercedes Lackey's Valdemar story setting. Tarma's tribe is all but extinct, and Kethry is expected to restart it. Which she does. Fanfic is somewhat discouraged by the fact that Kethry is quite enthusiastically straight, and Tarma is asexual by divine directive. They're also sworn Blood Sisters, with that same divinity sealing the deal.
Though that doesn't stop them from occasionally pretending to be lovers. And living alone together when Kethry's husband dies and all her children grow up. They do love one another, it's spelled out more than once - but they really aren't lovers.
Although their relationship is strictly divided by their rank and they never treat each other familiarly, Commisar Ciaphas Cain spends decades of his long life fighting by the side of his aide, Gunner Ferik Jurgen. Jurgen is unflinchingly loyal to Cain and Cain, by nature an intensely paranoid and self-obssesed man, describes Jurgen as the only man he ever fully trusts. Cain even gets quite annoyed on his aide's behalf that Jurgen is always left out of the stories that circulate about him and goes into shock when he thinks Jurgen has died- and that's only 13 years into their association.
Bernard and Helmholtz in Brave New World. Bernard gets intensely jealous when John forms an instant bond with Helmholtz, and Helmholtz thinks to himself at one point that Bernard's self-centeredness distresses him because he likes Bernard. Later on, Helmholtz manages to convince Bernard to agree to be exiled with him to a faraway island...off-page.
The Scarecrow and the Tin Woodsman, who, after Ozma takes over as Ruler of Oz, decide never to be parted and live together in the Winkie Country. (In different houses). Not like it stopped them from frequent visits to one another and a few more adventures.
The Cowardly Lion and Hungry Tiger would fall under this category as well, as well as Dorothy and Ozma in later books.
Diana Villiers tells Stephen in the second book that "anyone would swear you were married to that man".
Actually, Stephen does seem to make some subtle passes at Aubrey, but Aubrey thinks Maturin is just trying to discuss gay sex and is not interested.
Horace Harkness and Scotty Tremaine from the Honor Harrington series - they are 'always' invariably assigned to the same ship. We later find out that this is because Harkness hacked the Navy's personnel files. They also have hugged several times throughout the series.
Horatio Hornblower, predecessor to both the above and inspiration for Honor Harrington. Bush starts out as Hornblower's superior officer, but they earn each other's respect and when Hornblower gets promoted past him, he requests Bush as his second, and Bush happily accepts. They're joined at the hip for the next few decades, through Hornblower's two marriages. In A Ship of the Line, Hornblower has a minor Heroic BSOD when his ship is destroyed...and Bush loses his leg. In Commodore Hornblower, Hornblower thinks to himself that having Bush around is better than having a wife (he's been married twice at this point). Horatio has a much bigger Heroic BSOD in Lord Hornblower, when Bush makes a Heroic Sacrifice. In the films, Hornblower's HLP is Archie Kennedy, and it's possible that Executive Meddlingkilled Archie off in part because he was interfering with the development of the Hornblower-Bush relationship.
Gafinilan and Mertil in Animorphs book #40. Also arguably Marco and Ax, briefly, after Marco fakes his death.
Actually, it's established in Animorphs that Ax and Tobias are the Andalite version of Heterosexual Life Mates...except on his planet it's called "shorm" which translates to "tail blade"...meaning someone who you would trust to put their tail blade against your throat.
H.P. Lovecraft did this a few times. In The Hound, the two main characters retreat from the world to wallow in (chaste) decadence together. Poppy Z. Brite wrote a tribute to this in which the equivalent characterswere gay.
Another Lovecraft example: in Herbert West - Reanimator, West and the narrator live and work together for many years; if memory serves, the narrator even joins the military to stay near West during World War One. (And see under Film, above.)
Interestingly, following a failed marriage Lovecraft's considered the poster boy for abstinent asexuality.
Natty Bumppo and Chingachgook of The Leatherstocking Tales. Natty is even a sort of second father-figure to Chingachgook's son.
Fafhrd and the Grey Mouser, both shown as enthusiastic heterosexuals. In the last book Fritz Leiber wrote, the Mouser wonders why he and Fafhrd have never extended their relationship into the sexual, and whether this is a weakness.
Played with in Michael Chabon's novel Wonder Boys. Protaganist Grady admits that he hoped his best friend Crabtree would be his life partner, but this is complicated by the fact that Crabtree is gay. Ergo, the very straight Grady oftentimes finds himself jealous of Crabtree's boyfriends.
Jordan McKell and his partner Ixil from Timothy Zahn's sci-fi murder mystery The Icarus Hunt. Like Han Solo and Chewbacca they're both male, one human, one decidedly alien, and have spent many years flying a two-person ship. Apart from facilitating their murder investigation and flight from some particularly **** alien highjackers, their partnership turns out to be a major plot point.
Also, arguably, Artemis Fowl and his manservant, Domovoi Butler. Admittedly, it's just Butler's job, but the point is, Artemis is heterosexual (Butler is married to his job), they're partners, and they will be together for life. It's in Butler's job description.
Jim Pooley and John Omalley from Robert Rankin's Brentford Trilogy. In fact all of Rankin's heroes are straight maled and nearly all have a "bestest friend" whom they "love in a manly mannish way".
James Potter and Sirius Black in Harry Potter. The information of James' death was enough to make Sirius burst into crazy laughter, according to the Word of God 'because he knew what he'd lost'. He continues to mourn James throughout the rest of his life (not helped by the Dementors), and it is clear that James was the person he loved most in the world, with no notable romantic interests at all and no other friends reaching the level that James held in his life. They're also referenced as being 'like brothers' (though with the way Sirius felt about his own brother I doubt it is an accurate comparison). Although, the way Sirius speaks about James is obsessive and seems bordering on infatuation, which might argue against the level of heterosexuality in this particular life partnership.
One reason for this is that Sirius had big issues with his family and James was the only friend he could really confide in, being another pure-blood wizard. James also let Sirius live with him after Sirius ran away from Grimmauld Place. Friends like that are hard to come by.
The Weasley twins. Twins tend to be close anyway, but Fred and George did everything together. From birth to pranks, Quidditch, dropping out of school, going into business...you name it. Even when Fred appears to start dating a girl in their year, this is mentioned almost as an afterthought because Fred and George might as well have been joined at the hip. Word of God says that when Fred is killed in the last book, George never gets over it. He names his first son after him.
Parvati and Lavender are rarely seen apart, either. This also stands out because of the fact that Parvati has a twin sister as well - although she's only ever mentioned in passing.
Crabbe and Goyle if you really think about it. Up until the point where Crabbe got himself roasted alive by using magic he couldn't control.
Arthur and Ford in The Hitchhiker's Guide to the Galaxy. Ford constantly saves Arthur's butt, even when it'd be more convenient for Ford to simply take off and leave Arthur behind--surprising, considering that Ford by nature is incredibly selfish and self-serving. Having Arthur around only makes Ford's life more difficult--everything needs explaining (and Arthur takes a while to catch on), Arthur has almost zip self-preservation instincts (to the point that Ford has to drag him away from an army of killer robots), and (perhaps the most irritating thing for Ford) Arthur never seems to want to do anything ("We're popping off to a party and having a bit of fun. Is that an idea you can wrap your head around?") By Mostly Harmless, the two seem to be completely sick to the teeth of each other (to the point where Arthur seems to want Ford to kill himself), and yet they support and rely on each other anyway. If that's not hetero-love, I don't know what is.
One may notice that instead of becoming closer friends as the series goes on, bonding through hardship as usually occurs, the two like each other less and less as the books progress. In the first book, they seem to be close friends that consistently stick their necks out for one another. Ford also tolerates Arthur's cluelessness since it is understandable. Later on, they become estranged with the relationship. Then annoyed. Eventually they seem to truly dislike one another. And finally, they just sort of passively tolerate each other's presence. An odd-ball example of this trope. But an example nonetheless.
Caesar and Brutus start off as such in Conn Iggulden's Emperor series.
Brutus (or Marcus as he's known in childhood) was a ward of Julius' family when they were growing up. They were close as brothers, though Marcus considered himself The Unfavorite for the cool treatment he sometimes got from Julius' father. After they are reunited after Sulla's death, Brutus even surrendered command of his legion to Julius after they were reunited in order to honor The Promise. After this, though, the trope begins to be deconstructed. Brutus grows jealous of Julius' military successes, which he feels he could have won just as easily, as well as angry over Julius' romantic relationship with Brutus' own mother. During the campaign in Gaul, there is a tension in their relationship, which Julius taking Brutus' loyalty for granted and Brutus growing frustrated at living in Julius' shadow. At one point, while on leave in Rome, Brutus sleeps with Julius' sixteen-year-old daughter, whom he had known since she was an infant. When he returns to Gaul, the awkwardness of his friendship with Julius reaches even greater heights. All of this culminates during their triumphant march on Rome, when Julius selects Mark Anthony to stand as consul with him instead of his most trusted friend, neglecting to tell Brutus that it was because he wanted him to go with him to track down Pompey's army. This slap in the face is the final straw that drives Brutus to abandon Julius and join Pompey's forces. When Pompey is killed, Brutus is welcomed back by Julius, who remains torn because he wants so **** much to hate Julius, but can't bring himself to do it. They start off as Heterosexual Life Partners but Brutus keeps blaming Julius for more or less everything Brutus wants but doesn't get, ignoring the fact that Julius is the son of a senator and Brutus the son of a commoner and a prostitute
Gilbert and Isak, the "Mice" of Taylor Anderson's Destroyermen series.
Not quite. They're actually half-brothers.
Harry Cat and Tucker Mouse, of The Cricket in Times Square.
Lissa and Rose in Richelle Mead's Vampire Academy series. The two are psychically bonded to each other for life. In the fourth book, Rose goes off to kill Dimitri, leaving Lissa behind. The separation leaves a toll on both, and at the end, when Rose comes back, Lissa tells her to not leave her behind again.
Lissa: I just wanted to say, after everything that's happened...well, I don't want us to ever have this kind of separation again. I mean, I know we can't be together every single second - and that's kind of creepy anyway - but we're bonded for a reason. We're meant to look out for each other and be there for each other.
Ned Stark and Robert Baratheon used to be, in A Song of Ice and Fire. This being a series very far down the sliding scale, they both move away, get married to people they do not love (not at the time, anyway) and speak only once over the next 14 years (Ned has no interest in things in the south, while Robert is busy being king; it is notable that Robert's Hand was the mentor to both of them, and that once he dies, Robert immediately goes to Ned for the position despite the long gap since they were last together). When they finally do start hanging around each other again, the consequences are fatal for both of them.
Additionally, Ser Duncan the Tall and King Aegon V.
Tynian and Ulath grow to be this during the course of the series. In the second trilogy, someone explains their constant banter as a way of saying they love each other--and embarrasses all of them; men aren't too comfortable with the word 'love' when it's applied to other men (even if he IS your best friend).
Nero Wolfe and Archie Goodwin. Granted, Nero was Archie's employer, but they had a hard time generally without each other. Archie was a well-reputed ladies man throughout the 30+ years of novels, and Nero was a famed asexual, living only for fine food and his orchids.
Mr. Darcy and Mr. Bingley of Pride and Prejudice -- though it's more Darcy's doing things Bingley and he recognizes early on that Bingley is entirely too trusting and could easily get in with the wrong crowd if he's not careful. Thus, he starts the friendship with Bingley so that he can watch him and make sure he's making the right choices.
Elizabeth Bennett and Charlotte Lucas were this for a long time...until Charlotte got married. To a smug, idiotic, pompous windbag.
Leia and Winter are implied to be this in the Star Wars Expanded Universe, though their relationship isn't fleshed out very much since Winter tends to be one of the more-overlooked recurring EU characters. Technically, they are sisters by adoption but this detail wasn't established until fairly recently and they consequently tend not to refer to each other as such (presumably in part because they've always known they weren't biological sisters, even before The Reveal of Leia's true parentage).
Bran Tse-Mallory and Truzenzuzex from Alan Dean Foster's Flinx novels. Justified because they were once operators of a Humanx Commonwealth stingship, which required their minds to be linked so closely that their mutual understanding has never faded.
Locke Lamora and Jean Tannen, in Scott Lynch's Gentleman Bastard series. Either one of them would die for the other. And until we see how Republic of Thieves plays out, it seems Locke may have done just that.
Norathar e'Lanya and Cawti. They worked together as assassins until one of them turned out to be next in line for the throne and the other married a mob boss. Several years later, Norathar is helping Cawti to raise her child, in spite of the vast difference in social class and Cawti's anti-Empire political beliefs.
Outcast of Redwall's Sunflash and Skarlath, to the point that Sunflash writes sappy poetry about Skarlath.
Joseph Corrigan and Joseph Feehan, aka Corrie and Fee, from Blessed Are The Cheesemakers.
Joly and Lesgle in Les Misérables. Hugo specifically says they "hold everything in common", including Joly's mistress. Fanfic tends to portray them as a gay couple, probably due to the Ho Yay severely affecting the book's other male characters, but there was never any canon evidence that they were anything other than straight.
Tyl and Karl in Ursula Zilinsky's Middle Ground (though Tyl is bi leaning gay, there's no attraction there). Karl explicitly says he loves Tyl.
Rosemary Sutcliff, a celebrated English author of children's historical fiction, is all about this trope. Book after book of hers has as the main characters two guys who are lifelong best friends, often from childhood, who go everywhere together, fight and hunt side by side, hug each other, openly express love and devotion to one another, mingle their blood to become brothers, kiss each other's foreheads, cry in each other's arms. But they aren't lovers, both are implied to be straight though usually neither is shown having any affairs, and often one of them eventually marries without this disrupting the friendship. Examples: Drem and Vortrix in Warrior Scarlet; Lubrin and Dara in Sun Horse, Moon Horse; Artos and Bedwyr in Sword at Sunset; Marcus and Esca in The Eagle of the Ninth; Prosper and Conn in The Shining Company; Beric and Cathlan in Outcast; Justin and Flavius in The Silver Branch (they're also cousins of some sort); Simon and Amias in Simon; Thomas and Tussun in Blood and Sand; Randal and Bevis in Knight's Fee; Amyntas and Leon in A Crown of Wild Olive...you get the picture.
Tom Sawyer and Huckleberry Finn. Worth noting that in the first book Tom had a "bosom friend" named Joe Harper, but once Huck came in they became a trio, and then Joe was dropped altogether, probably because Huck was so much more interesting. At one point Tom hinted to Huck that he liked a girl and wanted to marry her, and Huck tried to talk him out of it, finally saying, "Only if you get married I'll be more lonesomer than ever." Tom's response? "No you won't, you'll come and live with me."
In Huckleberry Finn Huck is told by Mrs. Watson about Heaven, thinks it sounds pretty boring, and makes up his mind not to go:
I asked her if she thought Tom Sawyer would go there, and she said not by a considerable sight. I was glad about that, because I wanted him and me to be together.
Pooh and Piglet of Winnie the Pooh (who eventually move in together). A memorable scene involved Pooh deliberating (in poetry) who to visit for the morning, the verses beginning with, "I could spend a happy morning seeing X"; he ends up heading to Piglet's, which is what he was secretly planning on doing anyway, admitting that "I could spend a happy morning seeing Piglet/And I couldn't spend a happy morning not seeing Piglet."
This shows up in the work of Ross Thomas whose Mac and Padillo and Artie Wu and Quincy Durant series have this. Both are about life long partners, but are shown to heterosexual with one happily married (Mac, Wu) and one a Cassanova (Padillo, Durant) in each set.