I have been a fan of Tony Buzan's mind-mapping for many years, and have used several programmes for it, primarily kdissert and now freemind.
A mind map is a diagram used to represent words, ideas, tasks, or other items linked to and arranged around a central key word or idea. Mind maps are used to generate, visualize, structure, and classify ideas, and as an aid to studying and organizing information, solving problems, making decisions, and writing. Further information about mind-mapping can be found at http://en.wikipedia.org/wiki/Mind_mapping
Kdissert was developed by Thomas Nagy but is no longer in active development, but is still available in debian squeeze, but not in wheezy. It is very easy to use and quite complex documents can be created with minimal effort.
Semantik followed on from kdissert, and again developed by Thomas Nagy, but seems to have stalled in its development since August 2009. It can be found at http://www.freehackers.org/~tnagy/kdissert.html but is not available for debian squeeze or wheezy due to it requiring qmake and uic3.
Freemind is a java based programme and is available in debian squeeze and wheezy, and is in active development. It can be found at http://freemind.sourceforge.net/wiki/index.php/Main_Page. It is slightly more complex to use than kdissert but gives very elegant mind-maps which seem to be easier to follow. It is available cross-platform and produces consistently good results.
Another clone of freemind is freeplane which is available for squeeze and wheezy. It uses the same commands as does freemind and also saves its mind-maps with the same suffix, so they can be read and used by either programme. It can be found at http://freeplane.sourceforge.net/wiki/index.php/Main_Page.
For myself, I'm having to get used to using freemind/freeplane instead of kdissert, but I think that the results look better and more professional.