I recently downloaded the Spore Creature Creator, available here. It's a free preview of the creature editor that will come with the game Spore, which I last heard was due to be released in early September.
The creature editor is free, though it's a bit buggy, and you have to have pretty high machine specs to run it. Here's one of my early creations:
The game itself sounds pretty cool. It's from the guy who designed the Sims games (e.g. SimCity, SimEarth, etc.). I never really got into those games, but they're apparently very well done and popular.
Spore is supposed to be a massively single-player game, in which you follow the trajectory of an ecosystem from microbes to advanced, space-faring life forms. I'm not sure how the creature creator fits into the gameplay, but they did an awesome job with just this piece, so I'm interested in seeing how the game plays.
It's also interesting because I've been thinking a lot more about how to compactly represent arbitrary 3D morphologies in such a way that allows for useful designs to easily emerge via mechanisms like mutation and crossover. I wonder if the creatures in Spore will be evolvable...I originally heard about the game as an a-life game, but I don't know.
Anyway, if you're machine has pretty good horsepower and a decent graphics card, have a go.