Drawesome.uy - Offers both Drawing & Guessing games as well as collaborative drawing lobbies.
iSketch - Isketch is a Free to play, Puzzle, Education, Word, Drawing and Multiplayer video game.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
LetsDraw.It - Guess and Draw, Drawing Battle, Pictionary, Copy picture - online drawing games where you can compare your skill with others.
Doodle or Die - It's like the game of Telephone. Except with doodles.
Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.