For finding new music, ages ago, mp3.com was the tops, until they made the mistake with the Digital Locker, got sued into oblivion, then the site languished. C-Net bought the name, and are trying a decent effort, although it seems hard to find stuff other than your usual major label stuff.
Soundclick.com has some new and decent stuff here and there.
Oddly enough, one of the better sites I've found for finding new bands, is MTV's URGE music store.
Anyone have any others? Of course, there is Myspace, but browsing there seems to be like trudging through the intake port of a water treatment plant, looking for the rare gem of a band.