FreeMind is a premier free mind-mapping software written in Java and therefore a multi-platform high productivity tool.
It claims that its operation and navigation is faster than that of MindManager (a commercial software only for Windows).
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 in study, organization, problem solving, decision making, and writing.
I highly recommend to install the latest release candidate 0.9.0rc3. Just unpack the archive somewhere and launch freemind with one of the startup scripts for your platform (and of course, make sure you have Java installed).
If you want to get automatic updates, use Ubuntu's package management.
echo "deb http://eric.lavar.de/comp/linux/debian/ unstable/deb-src http://eric.lavar.de/comp/linux/debian/ unstable/
deb http://eric.lavar.de/comp/linux/debian/ experimental/
deb-src http://eric.lavar.de/comp/linux/debian/ experimental/" > /tmp/freemind.list
mv /tmp/freemind.list /etc/apt/sources.list.d
aptitude install freemind
At the moment, this will install Release Candidate 1-2, not the latest one.