One of the sad things about the Web is that some brilliant old resources disappear. The Xmorphia website at Caltech was a great way of exploring the patterns a reaction-diffusion system could produce, an early landmark of web science visualization.
Fortunately Jonathan Lidbeck at University of Oregon has created this Java demo: 2D Gray-Scott Reaction-Diffusion. It is a great successor, with new features. After all, these days you don't need a supercomputer to run the simulations, they can be done in an applet in realtime.