Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
Sketcharama - Draw a given word and let the other players guess your sketch.
PaintTool SAI - Learn to draw in different ways.