miniPaint - Online graphics editing tool use HTML5 technologies like Canvas, Offline support, Drag and Drop.
GIMP - GIMP is a multiplatform photo manipulation tool.
RealWorld Paint - RealWorld Paint is an free portable image/photo/animation editor.
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.
Pro Paint - Paint for Mac Pro is the best Mac version of paint program used to edit image, filters and paint on Mac. A good alternative to Paint Tool Sai Mac.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.