Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Epic Pen - A windows tool for drawing over your desktop and applications
Sketch - Professional digital design for Mac.
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.
Affinity Designer - Professional creative software, exclusively for Mac.
PaintTool SAI - Learn to draw in different ways.