Heres a simple piece of software that some might find useful to generate arbitrary seamless textures. I personally use it to generate dirt masks, scratches and such over a base texture.
Description from their website
"Texture Processor is the unique program for creating various textures. The basic algorithm placed into the heart of Texture Processor allows to get practically unlimited quantity of complex textures in a minimal amount of time. Texture Processor is intended both for 3d-artists and web-designers. Besides, any other users not able to draw in various graphic editors, will receive full pleasure and breadth of actions, working with Texture Processor."
http://www.paurex.com/?products&prod=tp12