Great free tool for online mindmapping

I use a variety of mind mapping tools.  I use MindNode Pro on my MacBook and MindJet Mind Manager 8 on my PC.  On both machines I use FreeMind and, for me, that free tool is the best of them all.  What I have missed is the opportunity to do collaborative mind mapping with a clean and simple interface.  X-Mind is available for Mac, PC and Linux and has free and paid ($49 USD p.a) versions.  Click here to go to their website.

There's also a good review of mind mapping tools and apps here

Brian