I’ve written a plug-in called MathMap for the popular image manipulation program The GIMP long ago and have worked on it on and off ever since. Rather than doing a specific effect MathMap is a simple mathematical language which can be used to specify a huge number of different effects (here are some examples).
MathMap has been used to illustrate complex functions, to generate weird Mandelbrot fractals, to analyze X-ray images of crystals and now seems to be very popular on Flickr to create Escheresque images – there’s even a group for those images!
Photo by Josh Sommers
If you want to try out MathMap for yourself, get it here if you use Linux (or MacOS X and you really know what you’re doing). There’s also a MacOS X version with a much nicer GUI, and an unofficial Windows version.