OpenBoard - The OpenBoard software is an interactive white board software.
skribbl - Word guessing based on drawing.
Conceptboard - Instant Whiteboards for Teams & Projects
LetsDraw.It - Guess and Draw, Drawing Battle, Pictionary, Copy picture - online drawing games where you can compare your skill with others.
Scribblar - Scribblar is a free online collaboration tool that is perfect for online tutoring.
gartic.io - Draw, guess, and win!