The Wikipedia has a page about extranets. From a UK perspective, it helpfully lists all the members of the NCCTP, and includes links to all the NCCTP members, plus about a dozen other providers (some from the construction industry – such as Citadon and Constructware – some not, eg: ProjectFusion, Xerdict), and one or two other links.
However, there is probably still scope to improve the content and perhaps to collaborate on other pages linked to the topic. Last week, Ralph Grabowski was urging readers of WorldCADaccess to update the list of CAD suppliers. Perhaps there should be a similar Wikepedia list of project collaboration or extranet suppliers?