Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
FlockMod - Flockmod is a free real time online group drawing app with all the bells and whistles.
PaintTool SAI - Learn to draw in different ways.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
skribbl - Word guessing based on drawing.