When I was young I used to play "The Incredible Machine" all the time. I guess now, it's the obligatory Half-Life 2 and all related games. I play Garry's Mod all the friggin time, although I don't know if that really can be considered a game.
Unreal Tournament hands-down. This game has been around for nearly a decade and yet it's still one of the best first-person shooters ever made - more than that, one of the best games EVER made. Not only that, but the infinite modding possibilities extend the game far beyond what the developers could have possibly dreamed of. Truly epic.
Pretty much any modern strategy/1st person/3rd person shooter game, and some theme park games and Starcraft (I have the original one, and I'm surprised the servers are still up...).
For me it's diablo 2! But nowadays seldom got time to play games already, still awaiting for diablo 3 but I guess Blizzard is not making it as it definitely does not earn as much as WoW.
I always liked the command and conquer series. KOTOR one and two was really fun too. The incredible machine was a great game.... I'm thinking of all these old games... has anyone played the space/robot simluator Descent, it was pretty simple but a lot of fun?
As a maverick in a world that favors simulating the violent destruction of his fellow man to the development of his artistic and intellectual sensibilities, I will say that I prefer a good (as far as an adventure game can be good, I have seen some that are no better than GTA) adventure game, myself. An excellent example being the Apprentice Trilogy by Herculean Effort Productions: http://herculeaneffort.com/index.php?page=apprentice2