Freemind - a mind-mapping tool

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

Free Mind is a piece of OSS that allows you create interactive mind maps. Mind mapping is widely used in education and this software enables the creation of interactive mind maps that can be shared, for example on a web page.

Note that the current version is only 0.8.1 - which means it may not be completelty stable. However, as it only takes a few minutes to download and install and is easy to start using, you may want to try it out.