"A great, simple, collaborative mindmapping site. Invite other users to share ideas together in real time. It uses html which means that you can use it with most web enabled devices. Sign in using a Google account."
If you're using mindmapping and brainstorming in the classroom, this list from Mashable is updated and useful. There are a lot of new tools out (many of them paid but some not.) If your students are writing collaboratively, you should have them brainstorming collaboratively before they ever type the first letter of the paper or website.
"Organising information to find clarity is key for making good decisions. This collaborative mind-mapping tool allows multiple users to work together to put together their ideas and resources."
a free web application which offers the ability to create a colorful map for writers who like to color code the creative thoughts and processes. This tool also provides the ability to embed in an e-mail to send to publishers or other writers who may be collaborating on the project.