skribbl - Word guessing based on drawing.
Doodle or Die - It's like the game of Telephone. Except with doodles.
gartic.io - Draw, guess, and win!
Sketchful.io - Pictionary type game where each round a different player draws something that the other players have to guess the word for.
FlockMod - Flockmod is a free real time online group drawing app with all the bells and whistles.
Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.