Left 4 Dead 2
Battlefield Bad Company 2
Serious Sam HD: The First Encounter
Serious Sam HD: The Second Encounter
And a whole bunch of single players. I'm thinking of picking up TF2, Killing Floor, and maybe the original L4D...since it's still being supported. Picked up L4D2 when it was on sale, but I haven't put to much time in it to be honest.