There are a wide variety of mind mapping tools available online. They vary in functionality and pricing. Below you will find links to several that you may want to try. Be aware that many of the tools below require email addresses for student accounts.
Initially Prezi was developed as an alternative to presentation software such as MS Powerpoint. However, it works equally well as a multimedia mind mapping tool, and has the ability to link to online resources. Prezi also has free education accounts available to teachers and students. Multiple users can collaborate on the same Prezi.
Bubbl.us is an easy to use mind mapping tool. With a free account, you are limited to three mind maps. It doesn't do anything fancy, but it is simple to use and you don't need to worry about a huge learning curve. Bubbl.us also allows you to export your mind map as an image file or web page. Note: Create an account or sign in before creating a mind map that you'd like to save!
SpiderScribe.net is a powerful, collaborative mind mapping tool that allows up to three mind maps with a free account. SpiderScribe allows you to upload documents and photos.
MindMeister is another powerful mind mapping tool that allows up to three mind maps with a free account.