The Library of Congress has made its ID.LOC.GOV authorities and vocabularies service publicly available.
Here's an excerpt from the announcement:
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.