Mostly Fallout 3 these days - just as I was getting bored I found four of the expansion packs, so we'll see what that does for it! I've also got the Sims 2 and 3 going, but I'm not sure if I count as "playing" those... they're more for machinima and other creative work.
I also just located my old Diablo 2 CDs - one of the reasons I decided to come back to BF. Enough people seem to still play it that I might just get back in...