Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
GIMP - GIMP is a multiplatform photo manipulation tool.
FlockMod - Flockmod is a free real time online group drawing app with all the bells and whistles.
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.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.