Here's the games I play. Halo 3 Halo ODST Assassins Creed 1 and 2 Batman Arkham Asylum Star Ocean the Last Hope(I was going to get FF13 but alot of people said SOtLH is better so I forget about it :l) MW2(Who doesnt?) EDIT: Also I forgot about Command and Conquer 3 Tiberium Wars
Definitely Left for Dead 2, I was a big fan of the first one but just recently picked up the second one and it is great! You could probably pick up one new/used for cheap. Also Mass Effect 1 and 2 although I suggest playing the first one first, you can get that for cheap I'm sure.