Mathmap is a wonderful plug-in extension to GIMP which is based on idea of complex mathematical manipulation of images. You can create very imaginative filters with it. It gives you a very effective programming language which is used to combine filters and functions.
You can find it from Plugin Registry and directly from homepage. It is issued as Debian 5.0 package, which means that you have to install it with dpkg. However, before that you have to install some additional packages
sudo apt-get install libgtksourceview2.0-0
After that cd into the directory you downloaded the .deb package and
sudo dpkg -i mathmap_version_number.deb
Once installation is complete, you will find a vast array of new filters from GIMP filter-menu. They are categorized under entry Generic, where you see sub-entry Mathmat, which has its own sub-menus. You will probably spend days experimenting with them. One advice, it helps alot if you are familiar with non-linear mathematics, fractals etc.
With mathmap it is quite easy to do pictures like this