One of the most useful, and powerful open-source programs in existance is the GIMP. GIMP is an acronymn for GNU Image Manipulation Program. It was started basically because two Berkley students, couldn't find a good program to use so they made their own. Humorously enough, when they couldn't make the program look nice enough using motif, they decided to write their own tool kit. That toolkit, called the GIMP Tool Kit has been built upon, and is now the basic foundation of the GNOME project.
The GIMP compiles on most all modern unix systems, and win32 now. Incidentally, in case you were wondering about the price of the GIMP, its free. That's right, free. The GIMP development team are roughly 80%+ based in linux, which is a free piece of "Open Source" software just like the GIMP.
The following list of links I've found relatively useful and interesting.
That's all for now folks, but I'll add some more in a bit.