Mind mapping is an important part of research. As far I know, there are two open source options: freemind and xmind. I use both but prefer xmind because it is built on the Eclipse framework and is more elegant than freemind at the moment. If I need more functionalities such as exporting to Word or Powerpoint, I can upgrade to the professional edition with less than $50 per year.
I chose to install it on top my existing Eclipse instead of using the .deb binary file so that I do not have to leave Eclipse just for mind mapping.