Here's a very nice set of online games, hand-picked by web surfers and collected in one site with short explanations.
Great for passing the time (especially at work!)
[via ThisIsBroken]
One of the coolest ones (though you really have to try it yourself to appreciate it) is the Line Rider. You draw the lines a sled will ride on and then press play. When you're done, check out some amazing movies of what other people have done with this game.