Mindmeister is an online mind mapping and brain storming site that allows users to create and share mind maps. It has a free account, Basic, which allows you to create up to 3 mind maps and share, collaborate, and import data. If you want to use more than that, you would have to sign up for a paid version. There are discounts for education.
Mindmaster is full of features and very easy to use. There are mobile apps for iOS and Android.
Other Mind Mapping services:
Popplet - online presentation, mindmapping, and bulletin board
Spicynodes - visualize and organize information and resources
Bubbl.us - online brainstorming for free
Thoughtboxes - organize everything you do - great resource for education
