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.
Sketchboard.io - Visual Remote Teamwork. Solve problems and create ideas on endless whiteboard with your teammates by sketching.
draw.io - Online diagramming application
Draw.Chat - Anonymous real-time whiteboard with text and video chat.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
MultiDraw - Draw with others in real time.