Amnesia The Dark Descent, Penumbra Black Plague, Penumbra Overture (If you're into single player horror games)
STALKER series are really fun. Fallout 3, New Vegas?
If you're looking for multiplayer then Killing Floor is a good game. You shoot mutated shit with other people to survive.
Battlefield 3 is a really fun game; if you didn't get it yet. You can get ARMA 2 Combined Operations and play the Dayz mod. But I would wait for the standalone version (if it is ever going to come out)