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
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
Gliffy - Gliffy is a powerful HTML5 online diagram maker. Create a flow chart, network diagram, UML diagram, organizational chart or wireframe with ease.