SpiderScribe is an online mind mapping and brainstorming tool. It lets you organize your ideas by connecting notes, files, calendar events, etc. in free-form maps. You can collaborate and share those maps online!

looks like more than a mindmapping tool. You can link in pics, docs, pdf. Might be good for project work.

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Free Educational Technology: The 5 Best Free Mind-Mapping Tools for Teachers. How To Use The 6 Best Free Mind-Mapping Tools for Teachers

free mind mapping for students

18 Free Mind Mapping Tools for Teachers and Students