Diagrammr is an excellent site for making a diagram/mind map by just typing text. It really is so simple, all you have to do is type in your text and watch how the diagram develops. Another feature which I really like is once your image is finished you can either change the layout or embed into your site.
This is a definite resource I can see being used in the classroom and I highly recommend giving it a look.
To give Diagrammr a try click here.
For another detailed review, check out Larry Ferlazzo's great blog.