CinePaint is a free open source painting and image retouching program designed to work best with 35mm film and other high resolution high dynamic range images. It is the most popular open source tool in the motion picture industry — used in 2 Fast 2 Furious, Scooby-Doo, Harry Potter, Stuart Little and other feature films. CinePaint is used for painting of background mattes and for frame-by-frame retouching of movies.
Interestingly, their news page doesn’t show much activity since last year.
Here’s an article about how it was used for the Scooby Doo films. It details how Apple’s two-year-old purchase of the “other” two Linux film editing apps — Shake and RAYZ — may drive CinePaint’s development, because many suspect Apple will ditch Linux support.
Finally, O’Reilly has a nice profile of CinePaint, and why it’s worth looking into.