i've used xMind. QASymphony has some documentation-creating tools that work with web pages to automatically generate mind maps, you could look into that. (Full disclosure: We do a little work for them.)
Mind-Maps for QA
I am researching if our QA team should make the switch to mind-mapping tools. Do you use a mind-mapping tool? If yes, what tools have you used? What has your experience been and did it improve your team's effectiveness?