http://www.gimp.org/

The Gimp is the defacto standard in open source full-featured graphics editing applications.

The Gimp has been around forever, and is quite mature. The Gimp supports multilayer images, exports and imports many 2D raster formats, and comes with a multitude filters and plug-ins.

• supports multilayer, multi-channel images.
• can work with some Photoshop plug-ins.
• can create animated gifs.