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.
ibis Paint X - Drawn with + ibisPaint
GIMP - GIMP is a multiplatform photo manipulation tool.
Huion Sketch - Huion Sketch is an animation and drawing application that comes with all the tools and features, providing you ease when making a painting.
Paint.NET - Paint.NET is a free image and photo editing software.
Zen Brush - Zen Brush 2 is a drawing app focused on the strong yet beautiful feel of the East Asian ink brush.