Lambda the Ultimate

breve is a free 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.

I guess the on-topic bit is this:

breve simulations are written in an easy to use language called steve. The language is object-oriented and borrows many features from languages such as C, Perl and Objective C, but even users without previous programming experience will find it easy to jump in.

but frankly steve is probably not so interesting in itself; rather breve is just a lot of fun to play with.

Apparently, several AI/robotics groups use breve for their research, but it's very easy to get started and comes with a simple IDE and several demos, of which the most interesting is undoubtedly the Walker, a genetic algorithm which evolves four-limbed walking "plates".