Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.
Sketch Toy - A drawing application that lets you save and share replays of your work with friends.
skribbl - Word guessing based on drawing.
Sketcharama - Draw a given word and let the other players guess your sketch.
gartic.io - Draw, guess, and win!