DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
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.
PaintTool SAI - Learn to draw in different ways.
MediBang Paint - The official site for MediBang Paint, the free digital painting and manga creation software. You can download the latest version of MediBang Paint here, and get news and tutorials.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
Corel Painter - Corel Painter is media painting software.