Very cool Reaction-Diffusion tool in Java, made by J.I.Weichsel . The Wallpaper Machine Pretty cute Reaction-Diffusion page which produces some nice tiling textures. Haim Barad also gave a presentation on this subject at the 1998 Computer Game Developers Conference. Source code for a cellular basis function can be seen in the latest POV-RAY code. This is THE book on procedural texture generation, and this link takes you to it's home page.
Topics cover gasses, solid textures, noise and fractals.
C source is included on disk with the book. Grab it from Amazon, they've got both the 1st edition , and the new 2nd edition . A Procedural Approach , taught by Ken Musgrave. Check out the home page for the class, which includes many images and links. The result is natural-looking animal spots and stripes.