This is a tool I was introduced to thru a class I took at university.
"The CmapTools program empowers users to construct, navigate, share and criticize knowledge models represented as concept maps. It allows users to, among many other features, construct their Cmaps in their personal computer, share them on servers (CmapServers) anywhere on the Internet..." - The CMap web-site
I think it's amazing.
The updated version is here: CMap v4.02.