For the uninformed, the genius minds behind greats such as The Sims, SimCity, SimAnt, etc. have been working on an entirely new game. Spore. In Spore you begin as a single celled organism and over the course of your play time, your creature evolves to the point where it comes out of the water onto the land. Eventually your creature even begins socializing with other creatures forming alliances, or if it's more your style you can just kill the other packs/herds of creatures.
You'll go from controlling a single creature to a whole society of your creatures, all 100% customized by you along the way at each various stage of evolution. Where is this intriguing game you ask? It's not out yet. But! That doesn't mean you can't have a little fun in the mean time. I got my hands on the Spore Creature Creator a few days early, a little demo that takes you through the basics of creating a creature and lets you give it a test run and see all the little animations your new life form can do. All totally unique to however you design your beast.
I downloaded this at... 2:00am and here it is going on 4:30am and I'm still playing with it. You can't even do much else besides be creative and play around, but it doesn't even matter. It's still fun. I can only imagine how entertaining the complete game will be. So what did I do with my time? I created just a couple creatures to test out. Most notably: Mintoad. As if watching Mintoad hop around and dance wasn't cute enough I discovered a magical button that spawns baby versions of your creature... and they imitate anything they see mommy (or daddy) do.
If you're interested in Spore at all, you should definitely go snag the Creature Creator when it becomes available for public download.