XMind (Open Source Brainstorming and Mind Mapping Software)

XMind is an open source project that contributes to building a
cutting-edge brainstorming/mind-mapping facility, focused on both
usability and extendability. It helps people in capturing ideas into
visually self-organized charts and sharing them for collaboration and
communication. Currently supporting mind maps, fishbone diagrams, tree
diagrams, org-charts, logic charts, and even spreadsheets. Often used
for knowledge management, meeting minutes, task management, and GTD.

With its feature of open-source (except some professional features),
XMind makes it possible for you to integrate the software into your
product, store data into XMind files, or even build your own mind
mapping software based on XMind.

NOTE:
If you upgrade xmind from a older version, keep a backup of your settings:
~/.xmind/workspace/.metadata/.plugins/org.eclipse.core.runtime/.settings/
then remove the old file ~/.xmind and finally restore your settings after
the first launch of the upgraded xmind.