Hmmm, this is a thread I'll be keeping an eye on, I bet there's a lot of great PC games I've missed...
A couple I've enjoyed:
Call of Cthulhu: Dark Corners of the Earth
Great Survival Horror / FPS - Low spec hardware.
Great gauntlet / adventure-shooter - Up to 4 player!
It's new LaserSquad, and you can play on-line.
Return to Castle Wolfenstein
Infiltration (UT Mod)