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.
draw.io - Online diagramming application
BigHairyGoal - Get your ideas down and get insights on them.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
ClickCharts - Download ClickCharts free flowchart diagramming software to easily draw flowcharts and map out a visual representation of a data flow or other process.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.