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.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
MediBang Paint - The official site for MediBang Paint, the free digital painting and manga creation software. You can download the latest version of MediBang Paint here, and get news and tutorials.
Corel Painter - Corel Painter is media painting software.
PaintTool SAI - Learn to draw in different ways.
GIMP - GIMP is a multiplatform photo manipulation tool.