Mind maps are diagrams used to visually organize information. A mind map might not end up in the final GDD, but nonetheless is a great way to organize ideas and draft an outline of what will be written in more detail later. Game design is about solving problems and mind maps have proven a very effective tool across many disciplines. Every game designer should try to use them whenever he feels that simple writing or brainstorming is not helping to visualize on paper his ideas and thoughts.
Sometimes, including a mind map in the final GDD proves to be a great way to communicate the rationale behind decisions. This is particularly useful when working on collaborative documents or to inspire creativity in the reader: