fl0w / Jenova Chen / PC
fl0w has become popular as a Playstation 3 downloadable in the last year or so, but it got its start as a graduate thesis by a media design student named Jenova Chen. While the Playstation 3 version has some "enhancements", it also costs about 8 bucks to download, but the original PC version of the game is still freely available from Mr. Chen's website and is pretty much the exact same experience.
fl0w has you playing as a small worm in a big ocean. The goal is to simply float around and eat other living things, in order to grow and evolve, so that you can eat ... bigger and more complex things. I'm not sure if the game actually has an end, but you can traverse downward and upward through ocean levels, and you are able to see silhouettes of what is going on on the levels below you before you descend.
Along the way you will encounter other worms that are floating around, eating and growing like you are. You can try to outmanuever them and nibble their tails to steal their flesh for your own growth, but they may try to do the same to you. There are also "enemies" that pop up on certain levels - these are usually larger than you, and can be identified by the fact that they glow orange and rapidly pursue you when you get near them.
The game basically has a relaxed vibe, with simplistic controls (you use only the mouse to steer, and click or hold down the left button for a speed burst) and minimalist graphics and music to match. The ambient "oceany" tunes and the glowing stick-figure sprites are pleasant, and the game is a nice little brainless relaxation device for when you feel like playing something but don't want to have to focus too hard or expend too much energy.
The game runs in Multimedia Flash and is about an 8 MB download. An executable can be downloaded to play on your computer, or you can play over the web via a browser-based Flash version. It has also since been released for the Sony PSP.