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.
Open Peeps - A hand-drawn illustration library.
Autodesk SketchBook - SketchBook drawing and painting software provides designers, architects, and concept artists the tools to sketch ideas quickly and create beautiful illustrations.
Control Illustrations - 108 free flat illustrations with customizable characters
Clip Studio Paint - The artist's software for drawing and painting.
Craftwork - A collection of User Interface resources made by Craftwork