For me the first that come to mind are the Halo games with Grunt Birthday Party enabled, Borderlands, Fable 2 and Ninja Gaiden 2.

For some reason Grunt Birthday Party makes me try harder when playing the campaign. I get more headshots and laugh at the grunts running away and exploding into confetti when playing coop with my buddies.

Bizarre art with unimaginable weapons and affects? Borderlands is just great fun and I always find myself laughing out loud when playing coop with friends. It's quirky and the art style really let Gearbox mess around with the visual weapon effects which can be hilarious. Plus a midget with a shotgun? Classic!

Fable 2 is just so peaceful and the art style is so warm. I tend to just run around looking at the atmosphere with my dog and it's just a relaxing experience. 

Call me crazy, but there's nothing like skillfully dismembering a group of enemies at an unconscionable pace. And beating a tough enemy in this game is always rewarding and cheers me up with a sense of accomplishment.

How in the world could I forget Duke Nukem 3d? No explanation needed. 

So what are yours and why?

Lips, singstar...The kind of games that cheer me up...

Mario Kart Wii

Sometimes I'll leave even more angrier then before if a Blue Shell stops my victory, however.....

i play games to make me happy.

but i play games like god of war as a stress reliever and when i die because im bashing buttons i get stressed, so i go and play some wrestling :). 

Rhythm Heaven...always get a kick out of playing it

Little Big planet

Flower

Modnation racers

n

Perfect as a stress reliever

Puzzle games generally make me happy or cheer me up. I think a Proffesor Layton game would make me forget all my troubles.

Borderlands seems to be doing it for me at the minute.

CS 1.6, Halo3 and UT3. Nothing cheers me up like those games.