Krita

The big breakthrough in the new Krita 2.6 open-source painting application for professional (and wanna be) artists is support for the OpenColor IO color management system. A component of the Calligra graphic art and office suite for KDE, Krita is a robust, nimble and flexible painting application that makes creating art from scratch or existing resources a fun and productive experience. Krita is targeted at the needs of illustrators, concept artists, comic book artists, matte painters and VFX artists. The new version 2.6, besides hundreds of bug fixes and performance improvements all over the place, struts integration of the Vc library, expanded compatibility with the Photoshop file format, improved keyboard interaction and an updated default set of brush presets. Linux and Windows versions of Krita are available; a Mac OS version is in development.

http://krita.org