My Simple Surface is a new (beta) site for brainstorming or mind mapping. The way is done is by creating a surface or whiteboard that users can create and organize notes on. This site reminds me a lot of Inspiration and users of that should feel right at home w/ this. Once a surface is completed it can then be shared w/ others for viewing or editing.
I recommend checking out My Simple Surface by clicking here.
For my top 10 sites for creating a mind map or brainstorming click here.