A mind map is a diagram used to visually organize information. A mind map is hierarchical and shows relationships among pieces of the whole. It is often created around a single concept, drawn as an image in the center of a blank page, to which associated representations of ideas such as images, words and parts of words are added. Major ideas are connected directly to the central concept, and other ideas branch out from those. Mind maps can be drawn by hand, either as "rough notes" during a lecture, meeting or planning session, for example, or as higher quality pictures when more time is available. Mind maps are considered to be a type of spider diagram. A similar concept in the 1970s was "idea sun bursting".
In RPG games mind maps are a fantastic tool. GMs and players alike can greatly benefit from the usage of mind map software. It allows the party to create game notes that are linked together and gives a visual diagram of how everything fits together. This is also especially true for the GM.
We use Inforapid's Knowledge Base Builder. It's free to use, has nice features and is a bit more colorful in it's GUI than most mind map applications. Knowledge Base Builder