Exploratree is a great site for educators to use for creating mind maps or brainstorming. This is an easy site to use that allows for group collaboration and comments as well. Best of all, Exploratree has a number of different templates to choose from to find the right "mind map" for you.
Poplet is a great tool for creating collaborating mindmapping. It is similar to wallwisher/stixy, but with more of an emphasis on linked ideas. There is also a great app for the iPad which allows collaboration.