PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
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.
PhotoFlare - Quick, simple but powerful Cross Platform image editor.
Autodesk SketchBook - SketchBook drawing and painting software provides designers, architects, and concept artists the tools to sketch ideas quickly and create beautiful illustrations.
PixBuilder - Free Photo Editing Software for PC
Clip Studio Paint - The artist's software for drawing and painting.