Via defective yeti.
Society is a series of interactive environments each presenting its own set of objectives, where you must explore, learn, and adapt in order to progress. In each of the four stages, you are respectively an alien organism participating in the life cycle of hunting, growth, and reproduction; a translucent, headless creature navigating through a dark labyrinth; a swirling ball of lights navigating a musical soundscape; and a smooth pink ball arranging items on a grid in order to make a path between two points, a mechanical engineering game similar to Blueprint.
Each stage seems to be more abstract than the last, but it is the earliest two that I find the most interesting. You are subtly encouraged to set aside your values in order to approach existence from an alien point of view: as a pink blob of organs in a fleshy, womb-like environment, your two tasks are to catch and eat very cute furry creatures ("ruminants") in order to grow to adulthood, so that you may then enter the breeding grounds to get it on with four different partners and successfully reproduce; and in the next stage, in order to get past obstacles you must drive yet more cute, squeaky ruminants toward menacing black forms that swallow them in what appears to be a very unpleasant death. You are told to do this without pity or mercy, despite the wrenchingness of their cries.
In addition to the intriguing alienness of the whole thing, it's also a beautifully rendered environment, and a lot of fun. The site is French, with English translations for everything (except a couple of the commands in the music stage, which they must have forgotten to translate or something...but don't worry, you'll figure it out.)
Elu and its sequel, Elu Undersea, are two very nice life sim games by a favorite designer of mine, Autofish. They basically simulate an ecosystem in which various sorts of creatures spawn, grow, and interact. You can play as an omnipotent creature, eating and spreading seeds as you choose, or you can turn it off and just let the simulation play out. Cly5m's bold colors and smooth pixel art make this game an attractive treat, fun to play around with or just watch for a while.
These last three games are to download, btw, not play online...but they're small and minimal and no hassle at all, so you may as well.
Seeds from Iteration Games is the game that inspired Elu, and the idea is very similar. Seeds is much busier and more rapid, and a little harder to control, though you can manipulate the environment more directly, creating or destroying any creature. Another fun life sim.
Dexx Entertainment's Pixiquarium 2 is a neat alternative to all those simulated auqariums. In very(!) tiny pixels, you get a miniature aquarium which you can fill with any of thirty different species of flora and fauna, each with its own behavior profile that drives its interactions with the other species. Very, very tiny species.