Spore is coming sometime in September, but EA/MAXIS have released the creature creator inside the game ahead of schedule for us to goof around with. There is a free version and also a paid version ($9.99). The free version has about 1/4 of the available pieces to add to your creature, but otherwise it is the same thing.
In Spore, you will take a creature from single celled organism all the way to galaxy spanning civilization over the course of many generations and many different sub-games.