I just finished playing the game Portal from Valve (makers of Half-Life and Half-Life 2). Portal is set in the same world.
This is an AMAZING game.
For those of you non-gamers, the game is more puzzles then action, very tricky in some parts. It is filled with dark humor and the application of physics in game play (do NOT let that scare you).
You're not saving the world, you're not killing demons.
You're a test subject going through the motions... UNTIL... (spoilers).
If you have played this game, squee with me about it. If you have not, buy it or find someone to borrow it off of. It is lovely, sweet, great game, even for non-gamers, and it's not even that long in gameplay.
It's great. 