This is a Linux-based mindmapping tool I just found today -- wonderful! It's much faster than Freemind, and has far greater flexibility of arranging onscreen graphics. There is also a Mac OS X version, although I have not yet tried it.

http://www.insilmaril.de/vym

I also ran across this brief listing of Linux based mind mapping tools:

"freemind (great!), gjots2, hidam, hypernon, jreepad, kdissert, kmindless, knowit, known-text, notebook (tcl version), notemeister (this is way cool) spag, tkoutline (again, in tcl/tk) treecle, treeline, VYM (awesome), belvedere (a great learning tool and more!),wishlist (in tk, so it's portable) supernotepad (also in tcl/tk) and of course, the King of all outliners, TUXCARDS!!!!"