concept mapping

there's some really good usage of concept mapping out there... i'm thinking hyperphysics.
today i was looking up a term in the IUPAC goldbook and saw they had implemented some form of concept mapping and thought i should look into what was used. there are times that i think there should be concept maps for things but i have never taken the time to get software and try it out. from the examples it looks like a fairly functional program. so next time i think about making one i think i'll check out graphviz.