Freemind

Freemind is an open source "mindmapping" tool to help organise your thoughts and to provide a clear visual means of displaying ideas.

Currently, the module displays or exports a vocabulary, but I am planning views integration and more (even the craziest ideas are welcomed :) ).

Status update: since it is very difficult to edit a mindmap and send it back to the server, this module will do only visualization. If you want to do mindmapping with editing support, you probably want Graphmind.