The GIMP
If you haven't heard of one of the best and most popular open source programs, take a look. GIMP stands for GNU Image Manipulation Program. It has tons of features, filters, brushes, and tools. It supports many different types of files. It can be extended even further with plugins. This is one powerful image editor! This program has support for layers, a very powerful feature that many other graphics editors lack. Layers allows you to make you image into separate parts that can be worked with individually. It also does simple animations with the optional GIMP Animation Package.
