OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Synfig Studio - Just another WordPress site
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
GIMP - GIMP is a multiplatform photo manipulation tool.