Flow

This is a very simple, yet great and very addicting game. If you heard about the upcoming Will Wright game "Spore" and saw the demo-presentation. You'll find it has resemblances to the first stage of that game.
Part of the fun of the game is figuring out how it works. It's fairly simple and straightforward. Only pointer I'll give is that you have to eat the red-dot fish to go down a level and the blue-dot fish to go up a level.
Goal of the game? Have fun and grow as big as possible. When you beat the two largest fish on the deepest level you can start over as a jellyfish :D
Flow Game