Astah Community is a free software design tool which supports the latest UML diagrams.

Cacoo is a user friendly online drawing tool. Multiple users editing the same diagram means Real-Time Collaboration.

Dia is free and open source drawing software.

draw.io is free online diagram software for making flowcharts, process diagrams, etc

Diagramo is an online flowchart software. It's FREE and Open Source ( GPL ) It's pure HTML5 Share and collaborate.

Freeplane is a free, open source software application for creating mind maps and electronic outlines.

Bonita BPM is an open-source business process management and workflow suite.

Creately is a collaborative diagramming and design software used by teams around the world to communicate more effectively.

Diagram Designer is a vector graphics editor that allows you to create UML class diagrams, illustrations, flowcharts and slide shows.

Diagrammix is a tool for efficiently building attractive diagrams and flowcharts.

Edraw Max is comprehensive mind mapping and planning tool network diagrams, floor plans, workflow diagrams, business charts, and engineering diagrams.

Gephi is the leading visualization and exploration software for all kinds of graphs and networks.