You can check out this mind map here
Visual Root is a free web based tool to create collaborative mind-maps made from videos, images, and text. Visual Root mind maps are like threads in a forum, one person starts a mind map root, and other people add node responses. The size of each element of the map is determined by users up or downvoting individual nodes. There is a short tutorial on how to use Visual Root here.
It is a very interesting brainstorming tool. You can see at a glance which subjects are the most popular and how they are conected. It can also be useful for debates.
In my opinion one of the key features of Visual Root is the possibility to export and download each map as a FreeMind file, then you can edit the map offline with a mind mapping software, given that most of the mind mapping softwares are importing FreeMind maps.
Visual Root is also available as a Chrome App on the Chrome Web Store.