LetsDraw.It - Guess and Draw, Drawing Battle, Pictionary, Copy picture - online drawing games where you can compare your skill with others.
skribbl - Word guessing based on drawing.
gartic.io - Draw, guess, and win!
Cast-a-Draw - Pictionary style party game for Chromecast on Android
Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
Drawesome.uy - Offers both Drawing & Guessing games as well as collaborative drawing lobbies.