Freeplane is a powerful and free software for building mind maps. It is a redesigned version of the well known FreeMind, and is created by one of FreeMind's key developers. (GPL)
Cross-platform
VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts. You can use them for time management, to organize tasks, to get an overview over complex contexts, to sort your ideas etc. (GPL)
Linux
XMind is an open source project that contributes to building a cutting-edge brainstorming/mind-mapping facility, focused on both usability and extendability. (EPL)
Cross-platform
4 applications