MyPaint

MyPaint is a relatively new open-source alternative to Corel Painter and Art Rage. Unlike many open source programs, its designers are fiercely focused on usability and it shows. Unfortunately, as of writing, MyPaint has not yet been ported to Mac OS X (although they are looking for someone to do the port).

One of the nicest things about the MyPaint website is that they link to a whole bunch of similar (less mature) open source projects. The website is also notably better-designed than a typical open source website, and the forums show some signs of life. Good stuff.