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.
Clip Studio Paint - The artist's software for drawing and painting.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.