FreeMind is an amazing tool to create mental maps, brainstorm and organize your thoughts, meetings and projects. The best thing about this tool is that it is not only free but fairly simple to use. The software can be downloaded from their website. http://freemind.sourceforge.net/wiki/index.php/Main_Page
A 10 min video tutorial is all that you need to gain mastery on this software but even without this tutorial you would do fine.