Spore

I started playing Spore a little while ago, and it is truly a fantastic sandbox. You are charged with creating a species and guiding it from a cellular stage through to a star empire - and hand designing every facet of its development along the way.

These are screenshots of my own little creatures, the Palindrone - pink and poisonous with the ability to brainwash allies - and the Repogig (don't ask), which is a happy little predatory species that looks like a mix between a velociraptor and a Maori warrior. It's awesome.

Definitely check the game out if you have ever been interested in Sims-type games. (It's made by the same developers).