Osmos This is a FANTASTIC game. Download the demo and give it a try. →[More:] The page describes it pretty well (esp. the trailer, which gives you a sense of how cool the game looks), but the idea is that you're a "mote" in a sea of other motes. You drift around absorbing smaller motes, avoiding BEING absorbed by larger ones.

The trick is, to move, you have to expel matter one way to push the mote in the other direction, making you... smaller. If you're not careful, when you reach a target, you'll be too small to absorb it.

The music and bubbly graphics make it feel like you're playing inside a lava lamp. It's beautiful and very, very well-designed. And the full version is only ten bucks! (Currently for PC only, but they say Mac and Linux versions are in the works.)