Obviously Fallout 1/2, Silent Hill, C&C Red Alert.
As for things not mentioned before:
Dark Reign - The Future of War (Jeehun Hwang 1997):
A really solid classic theme very suited for the game.
FTL (Ben Prunty 2012):
A bit simplistic, but the vibe of space exploration suits the game perfectly.
Quake 2 (Sonic Mayhem 1997)
While heresy in most circles, I think this is actually more suited for Quake than Trent Reznor's take on it.
Vampire: The Masquerade - Bloodlines (VA 2004)
VtM:B's soundtrack also grew on me. A lot. Especially stuff like the Hollywood theme. Totally chillax. If you want to calm me down, just play this, I'll mellow out.