Drawesome.uy - Offers both Drawing & Guessing games as well as collaborative drawing lobbies.
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.
Doodle or Die - It's like the game of Telephone. Except with doodles.
PaintTool SAI - Learn to draw in different ways.
TwistedBrush Pro Studio - A powerful and unique natural media painting program.