Two amazing demos: Mooncheese and Spore
The two are quite unrelated, really, but definitely worth sharing.
You can see a recorded video of Mooncheese via Zwetan's blog, or download the files to run the demo on your computer (2Ghz+ machine recommended).
The second demo shows the next game from the creator of the Sim's: Will Wright demonstrating "Spore" at the GDC. It's a mix of Creatures, SimCity, Civilization. It starts at the cellular level and later gives you access to the macro level (creatures) and finally extends your scope to tribe, cities, planets and even galaxies.
All the content appears to be procedurally generated, based on the input of the player thru an editor (a cellular organism editor, creature editor, etc.). For example, your creature will animate itself based on its body structure. The behavior of your creature depends on its physiology and brain, as well as the interactive orders you give them (clicking the mouse around to accomplish various actions).
Although this is not an online game, your local environment will be enriched by creatures imported from other players games, that your creatures will compete against. This should hopefully extend the player's interest game, in addition to the level-oriented principle of the game.
See the Spore entry on Wikipedia as well as IGN's preview of the game for more info.