Currently, perhaps the best cross-industry sources of information on discipline-specific markup languages can be found at xml.org. Two sections on that site are useful: the catalog and the repository. The repository has echoes of the EDI approach.
In addition, the ebxml.org site has an embryonic document repository for e-business standards, which is likely to accumulate a significant number of relevant standards over the next few months.
A further source of such information, again in an early form, can be found on the http://www.biztalk.org Web site.
This type of activity is being carried out at a frenetic pace ...