Pearltrees is a service that combines social bookmarking with mind-mapping. Using Pearltrees you can bookmark websites and arrange your bookmarks into webs or mind-maps of related topics
If you're looking for a fast and responsive mind mapping tool for your students to use when they plan a project or diagram a concept, give Slatebox a try. Or if you just want to create organization templates for your students to use offline, you can use Slatebox for that too. Create your template, download it as an image, and print it out.
Stormboard is a slick service designed for hosting collaborative online brainstorming activities. Stormboard allows you to create an unlimited amount of "idea boards" or Stormboards with up to five collaborators on each one.