draw.io - Online diagramming application
Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
skribbl - Word guessing based on drawing.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
gartic.io - Draw, guess, and win!