Audiosurf, definitely. Even on a fast song, it absorbs and distracts you; slower songs are a good chillout experience that distracts you just the right amount. Peggle is also very relaxing when it's not being frustrating.
Portal.
Jets 'n' Guns, when you've already beaten it once or twice and the enemies stop being a problem and start being flying moneyboxes. Press the fire button from time to time and watch everything vanish under a flood of plasma and bullets. Ahhhh, catharsis.
Just Cause, because hilarious sandbox antics get the endorphins flowing. (Recipe for fun in Just Cause: get a fast car, drive around going at least twice the speed of the rest of the traffic, and sooner or later you'll accidentally crash into/run over enough people that the police start chasing you, and what with the bizarre driving physics that also seem to apply to the AIs, you'll find yourself laughing hysterically before you know it.)
Any Unreal Tournament, particularly 3 - stress relief at its best. Put the bots on a difficulty where they'll kill you and whatnot but not really be much of an obstacle to your inevitable victory, pick your favourite map, throw on a couple of not-quite-suitable mutators (try Rocket Arena on Morpheus with 16 players), and unwind through violence. Guaranteed to bring a smile to one's face.
