I was playing around with Photoshop CS to create random maps. Using some default brushes and tools (mostly clouds, difference clouds, hue/saturation, and levels), I created the following random map. I'm probably not going to get around to making any actual map modfiles, however, so if anybody wants to get creative with it, feel free. All told, I probably put 15 minutes into this. If somebody was detail oriented, they could probably make it alot nicer.
If there's demand for such thing and I find the time, I may perfect and automate the process into a photoshop action/macro.