GIMP - GIMP is a multiplatform photo manipulation tool.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
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.
Sketch - Professional digital design for Mac.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Adobe Illustrator CC - Adobe Illustrator CC is a commercial vector graphics editor.