Will Wright, the creator of Sim City and The Sims has a new game in development that I think looks about as cool as they come.

Imagine a game where you start off with a creature on a cellular level, where you float around eating things and eventually lay an egg.  This opens you up to the ability to make some changes to the creature and progress to the next iteration.  Further on you have a more complex creature in an ocean, swimming, eating and mating, which through the generations you refine to a land-based creature.  Eventually a descendant evolves sentience and you have tools and building, eventually going to a city level, a world level, solar system and galaxy.  And the entire game is created and descended from the creatures you evolve over time and populated with other creatures and civilizations from other players of the game.

There's a really cool video posted on Google Video that you can see by clicking here.  It's fairly long, but in it Will Wright goes through the levels of gameplay.

Pretty cool.

Spore Gameplay Video

Official Spore Website
The intro animation actually does a great and humorous job of conveying how far the game goes.