Flow is one of the most mesermizing and hypnotic Flash games out there, and it has a great life-simulator feel to it on top of that.
In the game, you play a small organism that, as it eats other organisms, grows longer and faster. Depending on how you play, your organism "evolves" to have a different look, with different placement and lengths of fins. Each level has a blue organism and a red organism. Eating the red organism dives you down deeper, with the background turning to black. Eating the blue lifts you up, with the background turning to a calming blue. As you descend, you encounter many other organisms which you can feast upon (or which can feast upon you!). The goal of the game, I think, is to reach the bottom blackness and see how your organism evolved.
The graphic style, done with calm, glowing circles and line segments, is simple and elegant. The depth levels are achieved with a blurring effect (thanks to Flash 9!) that resembles focusing on different items using a microscope. The audio reinforces the graphic style, and in the end, you have a very well-rounded, intuitive game that is deeply engaging. More Flash games should be this good.