DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.

Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.

FlockMod - Flockmod is a free real time online group drawing app with all the bells and whistles.

malmal - Draw together on a nearly infinite canvas with your friends or with strangers.

Anondraw - Draw and paint live with friends or strangers on a big canvas. There is also a gamemode and gallery.

skribbl - Word guessing based on drawing.