From an announcement message sent out by Sally McCallum, head of the Network Development and MARC Standards Office:
The Library of Congress has opened its ID.LOC.GOV web service,
Authorities and Vocabularies, with the Library of Congress Subject Headings (LCSH) as the initial offering. The primary goal of this service is to enable machines to programmatically access data at the Library of Congress but the web interface also provides simple user access. We view this service as a step toward exposing and interconnecting vocabulary and thesaurus data via URLs. For LCSH, we are fortunate to have been able to link terms to a similar service provided in Europe for RAMEAU, a French subject heading vocabulary closely coordinated with LCSH.
We are very interested to get feedback on the uses and usefulness of the service to inform ways that we might enhance it. (There is a comment form at the site.). Over the next few months we will also be expanding it to other vocabularies commonly found in standards that the Library supports such as the Thesaurus of Graphic Materials, geographic area, language, and relator codes, and preservation events and roles.
So please go to the site and explore it for yourself at http://id.loc.gov.