Apophysis is free program which can be used to generate mathematical objects called fractal flames. Fractal flames are mathematically-generated images which can sometimes be very beautiful. Here is an example of an Apophysis-generated flame:
As a side-project, I have created a DeviantArt account to host some wallpapers that I have created using these fractals. I combine two or more fractal flames together form Apophysis and then post-process them in Photoshop (hue, saturation, blending and size) to create a wallpaper. Feel free to have a look at them! If you would like a smaller resolution or a different aspect ratio, please let me know in the comments or contact me and I will provide them for you.
Check out the gallery here!