Here are a few to start with:
Metal Arms Page
(String Quartet - mp3s, sheet music, and cd art available)
(Lots of Games OSTs. Including Baldur\'s Gate, Fallout, Descent...)
Interplay Mp3 Site
01-15-04 - One more link to add to the list.
There is a link to one large zip file at the bottom of the page.
Diablo 2 Page