breve

breve is a free/open-source 3d environment for multi-agent simulations and artificial life, which can be used to simulate things like BZ reactions, evolution by natural selection, and the flocking patterns of birds (which by the way is a good example of how realistic behavior can be produced using a drastically simplified model).

Also on the site is the breveCreatures screensaver, a simple simulation of evolution by selective pressure which you can download on its own. Creatures are initiated as random configurations of moving blocks, and selected through successive generations for the most effective walking behavior. The video below shows the products of some other evolution processes in breve: