Hands down the best flash game i've ever played. The game is visually and musically very minimal, but this combination is very complementary and actually makes game play almost.... soothing. The game stays constantly engaging as you battle other species. You start out as a tiny eel and eat, grow and evolve. In your quest for food you encounter various creatures and dive deeper and deeper into the ocean. After eating the last morsel of food available, you return to the surface to begin again as a different species, a jellyfish. A downloadable mac or windows version is available without much googling stress.
The game was also released for PS3, which allows for play as more than just the two organisms the flash game allows. If I would have known this it would have definitely made an impact on my decision to buy an xBox360 instead of a PS3.