"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.
breve is available for Mac OS X, Linux and Windows."
I did a bit of experimenting with it, and this program is impressive! It has its own physics simulation engine, a 3D renderer and a powerful language, complete with interfacing capabilities to other languages.
I may post a script of one of the robots that are currently evolving on my (horribly slow) computer, if someone is interested.
To get an idea of the genetic algorithm in action, open one of the physics-examples in the demo section (walker being the fastest evolving one.)
My computer is waaaay too slow to do serious evolving (a G3@350MHurtz, sigh...) so I'm looking into a way to distribute the system... If I'm not gazing at the screensaver (OS X only)