MindMup is a web-based mind mapper that you can use free, with no limit on the number of maps. It supports multi-user, simultaneous access and, if you’re using MindMup’s free storage or storing maps locally on you computer, it requires no log in. This makes it an attractive tool for school use. That’s not to say that it’s a toy as it can be used for serious mapping.
See on www.mind-mapping.org