Freeplane

 Freeplane is a free and open source software application that supports thinking, sharing information and getting things done at work, in school and at home. The core of the software is tools for mind mapping (also known as concept mapping or information mapping) and using mapped information. Freeplane runs on any operating system that has a current version of Java installed.

http://freeplane.sourceforge.net/wiki/index.php/Main_Page