"In the simplest terms, concept maps are visual depictions of relationships. They are really useful for visual and non-linear readers like me! I've found that the process of creating a concept map (ie. identifying connections) allows for a better understanding of the material than making summary notes. "
http://www.ualberta.ca/~lbriggs1/briggs/openaccess.html
At some point someone was going to do a better job than me in bringing all the OA stuff together.
http://oad.simmons.edu/oadwiki/Main_Page
This is not exactly a directory but as Ina Smith correctly calls it the wiki family of tools: