breve

From the website:
"breve is a free, open-source software package which makes it easy to build 3D simulations of decentralized systems and artificial life. Users define the behaviors of agents in a 3D world and observe how they interact. breve includes physical simulation and collision detection so you can simulate realistic creatures, and an OpenGL display engine so you can visualize your simulated worlds."

Also:

"breveCreatures is a screensaver that simulates the evolution of virtual creatures in a physically simulated 3D world. Beginning initially with random creatures, the screensaver uses a genetic algorithm to develop creatures capable of realistic locomotion behaviors."