Wikipedia
Freeplane
Freeplane is a free, open source software application for creating mind maps (diagrams of connections between ideas), and electronic outlines. Written in Java, Freeplane is supported on Windows, Mac OS X and Linux, and is licensed under the GNU GPL version "2 or later".
In 2007, Freeplane was forked from the FreeMind project. Freeplane maintains partial file format compatibility with FreeMind - Freeplane fully supports the FreeMind file format, but adds features and tags not supported by FreeMind which are ignored on loading.