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.
gInk - An on-screen on-screen annotation software for Windows.
PaintTool SAI - Learn to draw in different ways.
Ardesia - Ardesia is an usefull software for use your desktop as a Sketchpad.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.