I've long been a fan of Gliffy, a fantastic online diagram tool that's like Microsoft Visio in your browser, only free. Now it has a bit of competition in the form of LucidChart, a similar browser ...

Lucidchart is a visual collaboration tool that makes drawing diagrams fast & easy. Learn how to create a flowchart in Lucidchart & import it into Word.

More than just mind maps, LucidChart makes it easy to collaborate on org charts, network diagrams, business processes, development workflows, and more.