The group who produced Scrivener just sent me an email about their new mind-mapping program, Scapple.
It's now available for both PC and Mac.
I have yet to use this and have no affiliation with them in any context whatsoever, except as a purchaser of Scrivener.
Here's a link for those who may be interested:
http://www.literatureandlatte.com/scapple.php
Don