I've just done a review of all the free web-based mind mapping applications like Mindomo, over at http://www.mind-mapping.org/web-based-mindmappers/
They all let you collaborate with others when making a mindmap.
The review details show what's free (and how much the subscriptions are for the others), what can import and export MindManager and FreeMind maps and, of course, where to find them.
Vic
http://www.mind-mapping.org
The master list of mind mapping &
information management software