I never really played a lot of PC games, but finally got around to a lot this year and have sunk a lot of time into Fallout 3, Borderlands, and am working my way through Assassin's Creed slowly but surely. When I beat AC I have to reinstall Star Wars: Knights of the Old Republic and beat it (I only have 40GB of space on that partition of my laptop, I need to re-partition after I figure out how to backup all of my save files).
Oh, and there is the Orange Box where I can always hop into games of TF2 with my friends sucking in a good number of hours...