I saw some really cool Fractal desktop wallpapers online earlier, they were created using Astrophysis free fractal generator.

By default it isn't able to produce large images (java is set to give it only 60m of ram). The secret to getting big images is to open the program using the terminal and send it the commands...

java -Xmx200m -jar apophysis-j-22.jar

The results a really satisfying (click on the image to view a larger version on my Flickr account).