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.
ibis Paint X - Drawn with + ibisPaint
GIMP - GIMP is a multiplatform photo manipulation tool.
Paint.NET - Paint.NET is a free image and photo editing software.
TwistedBrush Pro Studio - A powerful and unique natural media painting program.
Corel Painter - Corel Painter is media painting software.