MindMeister

MindMeister describe their tool as a "web-based collaboration solution that uses proven mind-mapping concepts to facilitate multi-user planning, brainstorming, and project management sessions all from within a standard web browser". 

MindMeister's benefits comprise ease of use, online and offline availability, real-time collaboration, and easy sharing. It innovates the way you take notes, brainstorm, or plan your projects. MindMeister — a solely browser-based software service — takes the hassle out of maintaining software installs for usage in organizations like schools or enterprises.

Main Benefits as described on the homepage:

Creativity and collaboration are reportedly the basis of every successful business, educational organzation and society in general. MindMeister is a web-based mind mapping and visual thinking application that delivers cutting edge benefits for everyone.

• Real-time Brainstorming: Simultaneously work with colleagues on the same map and see changes as they happen.
• Easy to use: simple, well-designed functions - no training required.
• Highest security standards: 256 bit data encryption, secure data center hosting, no administrator access.
• Mobile access: iPhone application, provides access to your online mind maps where ever you are.
• Open Standards: open source software. Easy and seamless integration into other applications.
• Access Anytime, Anywhere: Maps are stored online - no attachment mailing or version conflicts.
• Cost-efficient and easy to deploy: No need for local installation or upgrades, zero maintenance or compatibility issues.
• Offline capability: A offline Mode allows users to work on their mind maps even when they’re not connected to the Internet. All changes will be saved locally and synchronized back the next time the users goes online.
• Full Audit Trail: The entire change history of a mind map is stored as separate revisions or versions.

Editions:

• Free Edition: up to 3 mind maps, share and collaborate, import and export maps, publish maps on blogs and websites
• Premium [Aug 11 = 39$/year]: same as Free + unlimited mind maps, add file attachments, create and edit maps offline, search in maps, Advertising free, logo free exports, Integrated live chat 
• Business [Aug 11 = 6$/month]: same as Premium + Team administration interface, branded login and custom subdomain
• Academic [Aug 11 = 15$/year]: same as Business, but special pricing 

Link: http://www.mindmeister.com