Ok aside from YK-related acts and artists that have their own thread here (e.g. Eir Aoi, Linked Horizon), I find myself listening to these artists:
fripSide - really loving their songs for Railgun, even though I have not watched even a single episode of it, lol. After hearing them live in AFA Indonesia, they got me hooked on the song "-LEVEL 5- judgelight."
savage genius - first heard her in Pandora Hearts, started liking "Maze". Also started to like "Hikari no Yukue". Only listened to those two so far though, lol.
Some common western artists, but I do not keep them on my music player since I can hear them everywhere.