Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Online diagramming application
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.
PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Simplify and communicate complex information using data-linked diagrams.
