The Gentoo Wiki has a cool tutorial on how to create your own cool background images with gimp. Basically it is a guide to the flame renderer and some color mapping but the results are impressive. I think the title “fractal backgrounds” is misleading because I don’t think these are fractal image but with some experimenting they look quite good. My first try took me like 5 minutes and looks like this:
You can download the full-size image by clicking on the thumbnail. The article in the Gentoo wiki also has some great example pictures for download.