for unique stuff, head over to
www.garageband.com, they are mainly unsighed acts (though the guy who lost in that pop idol show has some tracks there) - it's all free stuff, though you might have to sign up for some bands mailing lists to get the tracks.
http://music.download.com/ - also has a lot of interesting stuff as well...
Free Downloads from Amazon
Music Plasma is also really awesome- -you can put in a band name that you enjoy, and it will map it out for other bands with similar styles...