MathMap is a freeware (Under the GNU GPL), node-based image processing application. It will work with GIMP and Photoshop. There are versions for WIn and Linux. There’s also a Mac OSX version but, the author states that it’s a bit outdated.
I just found out about Mathmap and a cursory glance looks as though it may be a real help in creating textures as well as a great tool for 2D art.
The MathMap website has download links and links to a demo video.
