Studio Pierrot is a Japanese anime production studio mainly known for shonen manga adaptations. They currently create the famous shonen anime Naruto, and its spin-off. They also produced the Bleach anime before it was prematurely canceled in 2012. Before the 2000s, they were best known for their Magical Girl series, such as Magical Angel Creamy Mami, Persia the Magic Fairy, Magical Star Magical Emi, Magical Idol Pastel Yumi, and Magical Stage Fancy Lala. They're also known for -among others- Dallos; the first OVA, Saber Rider and the Star Sheriffs, Yu Yu Hakusho, The Sonic The Hedgehog OVA and the Di C co-produced series The Mysterious Cities of Gold.
For a more complete listing, check out their ANN entry.