A free program from Tufts University -- concept mapping and
presentation software that's easy to use.
http://vue.tufts.edu/
"The Visual Understanding Environment (VUE) is an Open Source project
based at Tufts University. The VUE project is focused on creating
flexible tools for managing and integrating digital resources in
support of teaching, learning and research. VUE provides a flexible
visual environment for structuring, presenting, and sharing digital
information."