On The Brian Eno and Will Wright on Generative Systems video

I found it interesting how delicately balanced this program and pattern was. Since it was effectively an entire predetermined interaction then a simple change could destroy the symmetry in a matter of generations.

It also reminded me of the various "falling sand" flash games online these days. Not only will free falling particles (or sand) move down at a constant velocity, the different materials will intact with each-other differently. For example, fire will spread to a particle that it is touching of a material that burns. then the fire particle will go away and the other particle that is ignitable will be replaced with fire. This interaction continues over the next interval of time.

Here is one of the simpler ones out there:
http://chir.ag/stuff/sand/

On this particular game there is also a material called "???" which reacts in a chain reaction with itself and can form fascinating waves and patterns. This can be seen to the right.