By Subject

Babynames Thesaurus - You can search for a name from its meanings. The site has more than 10,000 names with meanings.
http://www.yourbabysname.com/thesaurus.htm

The Astronomy Thesaurus - Astronomical terms. Searches may be performed in English, French, German, Italian, and Spanish.
http://www.mso.anu.edu.au/library/thesaurus/

Library of Congress Classification Schedules - A list of LOC Classification schedules.
http://www.loc.gov/catdir/cpso/lcco/lcco.html

Art and Architecture Thesaurus - Getty's terms, as used in several AAT indexes.
http://www.getty.edu/research/conducting_research/vocabularies/aat/

Inter Active Terminology for Europe (IATE) - European Union terminology. Available on most European Union languages.
http://iate.europa.eu/iatediff/

Getty Thesaurus of Geographic Names - Structured vocabulary of place names.
http://www.getty.edu/research/conducting_research/vocabularies/tgn/

Maths Thesaurus - Definitions of common mathematical terms, along with broader, narrower, and related concepts.
http://thesaurus.maths.org/

Thesaurus of Musical Instruments - Over a thousand instruments, with relational links based on the Library of Congress Subject Headings.
http://alteriseculo.com/instruments/

Library of Congress Thesauri - Links to multiple LOC thesauri, including GLIN, the Global Legal Information Network.
http://www.loc.gov/lexico/servlet/lexico

Concept Space - Management education thesaurus from the London Business School organizes terms into a solar system of related ideas.
http://conceptspace.london.edu/

NASA Thesaurus - National Aeronautics and Space Administration Thesaurus. Includes coverage of scientific terms.
http://www.sti.nasa.gov/nasa-thesaurus.html

Alcohol and Other Drug Thesaurus - AOD - National Institutes of Health thesaurus. Provides resource for medical, social, and demographic terms related to alcohol and drug use.
http://etoh.niaaa.nih.gov/AODVol1/Aodthome.htm