GIMP Flame Fractal Backgrounds
I found this tutorial on how to create fractal backgrounds with GIMPs Flame filter on the front page of the gentoo wiki site.
You can actually get some very good results quite easily.
Here are the first few I managed to create.
Large versions are available here
Once you generate a good flame pattern you can keep creating new images and reusing the last flame settings but with different colours.
There is one issue. It appears that it has trouble generating large flames. 1600×1280 worked for me, but I could not get 3200×1080 working (dual monitor wide flame).