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.
Epic Pen - A windows tool for drawing over your desktop and applications
GIMP - GIMP is a multiplatform photo manipulation tool.
Paint.NET - Paint.NET is a free image and photo editing software.
PaintTool SAI - Learn to draw in different ways.
TwistedBrush Pro Studio - A powerful and unique natural media painting program.