PaintTool SAI - Learn to draw in different ways.
GIMP - GIMP is a multiplatform photo manipulation tool.
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.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.