The Gimp graphics program for Linux and Unix

The GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software suitable for tasks such as photo retouching, image composition and image authoring.
It is an extremely capable piece of software with many capabilities. It can be used as a simple paint program, a expert quality photo retouching program, an online batch processing system, a mass production image renderer, a image format converter, etc.

GIMP is extremely expandable and extensible. It is designed to be augmented with plugins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
Though a good (free!) alternative to Adobe PhotoShop on Linux/Unix systems, I have come to know that the Windows version of the GIMP is not very stable.

http://www.gimp.org