Bundled into the Half-Life 2 boxset on the 360, PC, and PS3 is a brilliant little game called Portal. The idea is that you use a gun to shoot one entrance portal hole and one exit portal and work your way around a set obstacle courses by travelling through these portals.
Here's the trailer which gives a few simple example puzzles:
After finishing the game, you recieve the best videogame credit song I've ever heard. Watch it here via Youtube or download the MP3 here. Some call it the best song ever. I might be inclined to agree with that.
There is also a Flash version of game available to play and it's actually quite decent. It's nowhere near as perfect as the real game, but it's still a better way to waste half an hour than reading PerezHilton.com or something. Play it here (full screen mode here).