There are some options for making collaborative concept maps. Webspiration is an online version of Inspiration (and Kidspiration). It's currently still in its public beta testing, but it looks like they are moving towards a subscription based model for their product. Most of the functionality of the full Inspiration software is available, and it is fairly easy to share, import, and export documents.
An alternative to Webspiration might be to use the shared "Drawing" type of document available in Google Docs. It requires a little more manipulation to link shapes (ideas) to one another than Webspiration, but the canvas is really flexible and with a little time, users can get used to making the interface work for them.
Header photo by Robert S. Donovan