Gimp/Pixelmator

If you have not heard of Gimp, it is a free image editor similar to Photoshop available here. It is very useful and is multi-platform.

I do a little bit of image editing here and there, for instance the banner up top or some desktops that I have created with some of my programs then make a little better with gimp. The mathematical properties of filters and image processing are interesting. Below are some images edited in gimp.