Astronomy Thesaurus - covers English, French, German, Italian, and Spanish.

CAB Thesaurus - multi-lingual coverage of life sciences, technology and social sciences.

ERIC (Educational Resources Information Center) Thesaurus - based in USA, covers bibliographic citations and digests, sponsored by the U.S. Department of Education.

Ethnographic Thesaurus - based in USA; covers folklore, ethnomusicology, ethnology, and related fields.

European Thesaurus on International Relations and Area Studies - based in Europe; covers international relations and area studies, and the languages English, French, German, Spanish, Italian, Polish, Czech, Croatian and Russian (English or German interface).

Getty Art & Architecture Thesaurus - based in USA; covers fine art, architecture, decorative art, and material culture.

Getty Thesaurus of Geographic Names - covers all nations of the world, including vernacular and historical names, coordinates, place types, etc.

Getty Union List of Artist Names - based in USA; covers artists and architects, including variant names, pseudonyms, and language variants.

LC Thesaurus for Graphic Material   - covers terms terms related to graphic materials.

Medical Subject Headings (MeSH) - covers medicine and health.

Public Health Language - based in the UK.

Thesaurus for the Social Sciences - based in Germany, covers terms in German - English - French.

UNESCO Thesaurus - covers "the fields of education, culture, natural sciences, social and human sciences, communication and information"

Visual Thesaurus - a commercial site that gives a sneak preview of a potnetially very useful service. The main service is not free unfortunately.

See also: Taxonomies & Controlled Vocabularies SIG of ASI .