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 software can be used for mind mapping and analysing the information contained in mind maps. Freeplane runs on any operating system that has a current version of Java installed.

You can find more information and a download link at http://www.freeplane.org/.

Freeplane has a lot of functionality and options. This wiki gives some [Tips] with regard to using Freeplane functionality in conjunction with GTD Sync.