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.
wxWidgets - wxWidgets: Cross-Platform GUI Library
TwistedBrush Pro Studio - A powerful and unique natural media painting program.
Electron - Build cross platform desktop apps with web technologies
Corel Painter - Corel Painter is media painting software.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.