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.
TwistedBrush Pro Studio - A powerful and unique natural media painting program.
GIMP - GIMP is a multiplatform photo manipulation tool.
Corel Painter - Corel Painter is media painting software.
Paint.NET - Paint.NET is a free image and photo editing software.
Artrage - A simple but lovely natural media painting and sketching program.