http://bubbl.us/

bubbl.us is a website I have found that allows anyone to create mindmaps and concept maps online for free. These can then be embedded into blogs or websites and can be added to or deleted from as needed.
The best thing about bubbl.us is that you can allow others access to the mindmap so they can edit and add to it as well meaning that it could esily be used with cooperative group work. A really great tool for teachers in the classroom.

http://bubbl.us/