I recommend I've Sound (Kotoko, Kawada Mami+Others) as well. The singers are very talented, able to handle anything from slow songs, to rock and superhyper denpa (Princess Bride :O). Mostly trance-inspired awesome.
My I've Sound folder is 12GB by the way, so yeah, I'm a huge I've freak :p

Blasterhead if you like heavy bass and techno.
Asian Kung Fu Generation for JRock.
UNDER17 for cutesy songs.