Threddie is cool site for brainstorming that uses a post and comment system that is not unlike Google's Wave thread discussion, as it is done in real time. While this site might not be as visual as other brainstorming tools, it is effective in its simplicity.
I highly recommend checking out Threddie by clicking here!!
Below is a demo video...
Check out my top 10 sites for brainstorming by clicking here.