Since most of the real stuff has been posted, figure I'll throw some remixed/tribute stuff out there. Machinae Supremacy's a weird ass band (I mean let's face it, pseudo metal with C64 sounds embedded in it is just plain weird) but some of the remixed snippets of older songs are pretty solid.
http://www.youtube.com/watch?v=pcpsVf2bOaE
Should be able to pick up a ton of old C64 tracks on there, some that are newer, I think in one of the later tracks there's some FZero, Castlevania, Megaman, etc, but not on the particular one I linked. Fun band as a diversion if you're into that kinda crap.
In a non 'classic' vein, the Silent Hill OSTs are simply amazing at times. Some of the tracks on there are absolutely fantastic and don't really seem like they belong on a game soundtrack. They're also pretty damned diverse as there are vocals in I think 3 different languages between them all and there are a ton of instrumental only that are all over the place. Couple links for those. As far as fitting the mood of the game/environment....I think this series is kinda unmatched.
http://www.youtube.com/watch?v=_cOYIpwll_M
http://www.youtube.com/watch?v=_qasnp026MY
http://www.youtube.com/watch?v=kJtz-...eature=related