www.spore.com
I'm sure most people have heard of Spore, but just in case...
It's from Will Wright, the smarty pants behind the Sims and Sim City and Sim This and Sim That. Spore is pretty much the ultimate sim game. You start as a little single celled organism, feeding off of smaller beings. After a while, you start to evolve. The best part is, you choose how you evolve. You create your creature, and eventually your race, from scratch. Want three legs and a mouth on top of your head? Do it! You evolve into a land dweller, where you will eventually proporgate your species, and then control your whole race as you build cities and conquer other races! You really need to check out the website for the full effect.