The American Historical Association has announced the release of ArchivesWiki, which is "intended to be a clearinghouse of information about archival resources throughout the world. While it is primarily designed to be useful to historians and others doing historical research, we hope that researchers in many disciplines will find it useful."
Basic information about more than 100 archival repositories (full list) has already been uploaded to the wiki, which is free and open to anyone (after a short registration process).
If this fleshes out well, I think it has tremendous potential.
Link added to the sidebar.