From the website:

MindMeister brings the concept of mind mapping to the web, using its facilities for real-time collaboration to allow truly global brainstorming sessions.

Users can create, manage and share mind maps online and access them anytime, from anywhere. In brainstorming mode, fellow MindMeisters from around the world (or just in different rooms) can simultaneously work on the same mind map - and see each other's changes as they happen. Using integrated Skype calls, they can throw around new ideas and put them down on "paper" at the same time.