Social Source Commons

Creately offers a clean, intuitive interface and mild learning curve. The free version lets you collaborate with a maximum of five other users, and any diagrams you export include the Creately logo. Creately lets you name your price for its professional version (minimum $1 a month), which removes the Creately watermark and lets you work with an unlimited number of collaborators.

This toolbox accompanies TechSoup's article on diagramming tools. Feel free to leave a comment or add your favorite tool.