flOw is a Flash game created by Jenova Chen and Nicholas Clark. You start off as a tiny creature swimming in a deep pool, and as the game progresses you get bigger and go deeper.
The game has nice sound and graphics, but the most notable aspect is the gameplay, which naturally unfolds and becomes more complex as new features and gameplay elements are introduced. There are no instructions that you need to read before you can play—you just get thrown in. New features and gameplay elements are introduced in a very natural way.
flOW is a short but rewarding game, and I recommend playing it.