malmal - Draw together on a nearly infinite canvas with your friends or with strangers.
draw.io - Online diagramming application
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
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.
Aggie.io - Draw a picture together with your friends in real-time over the internet in your browser.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.