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.
BigHairyGoal - Get your ideas down and get insights on them.
draw.io - Online diagramming application
StarUML - StarUML
Dynamic Draw - A Powerful idea processor.It's Open Source and Freeware.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.