Diagrammix - Tool for efficiently building attractive diagrams.
draw.io - Online diagramming application
ProcessMaker - ProcessMaker is an easy to use BPM and workflow software solution. It is used to design, automate, and deploy business processes of any kind.
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.
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.