CinePaint began as a fork of the GIMP more than a decade ago, but only now has reached the version 1.0 milestone. However, there isn't a whole lot of changes to this new version. "This release of CinePaint is numbered at 1.0 rather than 0.27 for administrative reasons. It is not significantly different from 0.26. It has known bugs."
Currently the CinePaint 1.0 release is only available for Linux with the Mac OS X and Windows builds not being available at present.
This software that's been used in multiple feature films is licensed under the GNU GPL.
CinePaint 1.0 can be downloaded at CinePaint.org.