Serious Sam 3: BFE can be purchased for $10 (75% off) from Steam. The game is fully working on Linux via the Steam beta client. This special promotion offer will end on December 10.
Killing Floor is a co-op survival horror FPS taking place in the devastated cities and countryside of England, after a series of cloning experiments for the military go horribly wrong.
Windows version of Killing Floor can be purchased from Gameolith for $9 (50% off). The package includes a Steam key which can be redeemed on Steam Linux beta client. Numerous users have reported that game is fully working on Linux. Check out a video:
You can also get Left 4 Dead 2 for 75% off on Steam. The game is not yet playable on Linux but Valve has already announced a Linux port. So if you want to add it to your Steam Linux collection, this is the best time.