The next Collaborative Web application that I'm not familiar with is MindMeister.
MindMeister is an online collaborative mind mapping tool. It's similar to Inspiration, a visual learning computer application which can be purchased and installed on a computer. MindMeister is so much more! Not only is it available online 24/7 at no cost, but it allows for real-time collaboration. Colleagues can brainstorm on a project simultaneously, view each others edits, and even integrate the use of Skype for discussion.
Here is a map that I created about my learning experiences while working on the csla tutorial. If you'd like to collaborate on this mind map please leave a comment so I can add you as a collaborator.