RAMON -- Eurostat's Classifications Server
This site, run by Eurostat, describes classification systems and other metadata
entities, such as " concepts and definitions, legal acts, methodological
manuals, national methodologies, theseaurii, etc." Numerous systems are
described in great detail, and they can be downloaded in various formats. In
addition, many "correspondence tables" are available to connect the different
systems. Finally, they offer various links in this area.