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!)
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.