Epic Pen - A windows tool for drawing over your desktop and applications
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.
PaintTool SAI - Learn to draw in different ways.
GIMP - GIMP is a multiplatform photo manipulation tool.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.