http://zeus.pin.unifi.it/cgi-bin/twiki/view/GIcatGI-cat is an implementation of a distributed catalog service, allowing clients to discover and evaluate geoinformation resources over a federation of catalogs, by a combination of query criteria (e.g. spatio-temporal extent, keywords, data source).
https://sites.google.com/site/geosspilot2/Home/clearinghouse-catalogue-registry-metadata