Although I'm more of a CD person, I've currently found:
Hayao Miyazaki
A MUSIC BOX HEALS YOU: Hayao Miyazaki (Greatest Hits?)
Ishida Yoko
Akano Seijaku (Shakugan no Shana ED2)
Kageyama Hironobu
CHA-LA HEAD-CHA-LA -2005 (Cover of the opening to Dragon Ball Z)
Kawada Mami:
Radiance (Starship Operators OP/ED)
KOTOKO:
Being (Shakugan no Shana OP)
Chercher (MariMite ED)
Gurasuno Kaze (Compilation)
Hayate no Gotoku (Hayate no Gotoku OP)
Kireina Senritsu (MariMite OP)
Oboetete iiyo (Mahou Shoujo Tai Arusu)
Real Oni Gocco (Real Oni Gocco)
Re-sublimity (Kannaduki no Miko)
Shichitenhakki☆Shijoshugi! (Hayate no Gotoku OP)
羽-Hane- (Not anime related)
Kushida Akira:
Kinnikuman Go Fight ! (Kinnikuman)
Mell:
Red Fraction (Black Lagoon OP)
Virgin's High (Sky Girls OP)
Proof (Hayate no Gotoku ED)
Mosiac.wav (Doku-Denpa group that does mostly eroge but also did the opening to Kyouran Kazoku Nikki and Sumomo)
Falsie the Pseudo-Science Girl
Future-Fiction:AKIBA-POP!!
Kimi Wa Nan Terabyte?
Last Battler Akibattler μ
Magical Hacker Kurukuru Risk
Megane Dene!
Space Akiba Pop (A bunch of eroge songs)
Spam Mailing Girl
We Love AKIBA-POP!!
Re-vo & Haruka Shimotsuki
Kirino Mukou ni Tsunagaru Sekai (Atelier Iris 3: Grand Phantasm OP)
Yoko Takahashi:
Yoake Umarekuru Shoujo (Shakugan no Shana OP)
Yuria
Sono Mukou no Mukougawa OST (Game)
Names are according to how iTunes lists them (as much as possible). All right. That's all I can find for now. I'm in the USA so no promises elsewhere. While the categories are rather poorly defined, iTunes does have one for anime, for the record.