In my list of must have applications is Freemind - an open source mind mapping or concept mapping program. It does a good job but as with the nature of open source a couple of people thought they could improve on it - So here are two derivitives of the original.
Freeplane is free and open source software to support thinking, sharing information and getting things done at work, in school and at home. The core of the software consists of functions for mind mapping, also called concept mapping or information mapping, and tools for using mapped information. Freeplane runs on any operating system on which a current version of Java is installed and from USB.
SciPlore Mind Mapping
SciPlore MindMapping is the first mind mapping tool focusing on researchers’ needs by integrating mind mapping with reference and pdf management. SciPlore MindMapping offers all the features one would expect from a standard mind mapping software, plus special features for researchers.
Have a look at the video and you will see how useful this is to high school students and those at university level.