Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.
Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
LetsDraw.It - Guess and Draw, Drawing Battle, Pictionary, Copy picture - online drawing games where you can compare your skill with others.
Panel Jam - Collaborate with others on comic strips