GIMP - GIMP is a multiplatform photo manipulation tool.
Talent Pool by Minty - Reach award-winning illustrators for your projects
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.
VPaint - Experimental vector graphics editor.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Procreate - Procreate is the most powerful and intuitive digital illustration App available for iPad®.