Almost everyone has mind-mapped before, whether or not the process was defined as mind-mapping. Since elementary school, we have learned to diagram our ideas and concepts, based on importance, around a central idea or concept, often connecting ideas through multiple branches. Unlike other forms of brainstorming, mind-mapping allows for greater freedom and helps visualize the overall concept. Here is an example of a basic mind map followed by a more intricate one:
Up until now, mind-mapping was limited to pen and paper. Online, however, I found several different softwares and applications for electronic mind-mapping.
This wikipedia article lists free and proprietary software: http://en.wikipedia.org/wiki/List_of_mind_mapping_software
I found these two free applications on the web: