Here's an interesting bit of Python code I hacked together – it's a script that takes an image and warps it so that it is tileable (making it suitable for a repeating backgound or a texture in a game).
A Mandlebrot fractal
If you use it on a photograph, it will come out looking like a fair-ground mirror. But it works well when applied to a pattern, or something more abstract, such as the fractal image on the left.
The code is public domain – use it for whatever the heck you want!
Update: Here's another, more interesting example, The original is here.