I just stumbled upon an interactive list of cool collaborative tools, organized by category. Very cool and obviously a lot of research has gone into it. I encourage you to check it out: http://www.mindmeister.com/12213323 

Also, if you haven't discovered mindmeister, where the the list is located, you are missing out on one of the best creative, brainstorming, mind mapping tool online.

Check out the Collaborative Tool Mind Map.