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

NASA Thesaurus

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

CAS General Subject Vocabulary Helper

Extensive listings for chemistry. Produced by Chemical Abstracts Service, a division of the American Chemical Society.
http://www.cas.org/vocabulary/index.html

Maths Thesaurus

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

Art and Architecture Thesaurus

Getty's terms, as used in several AAT indexes.
http://www.getty.edu/research/tools/vocabulary/aat/

Eurodic Autom

European Union terminology. Available on most European Union languages.
http://europa.eu.int/eurodicautom/login.jsp

Getty Thesaurus of Geographic Names

Structured vocabulary of place names.
http://www.getty.edu/research/tools/vocabulary/tgn/index.html

Library of Congress Thesauri

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

Library of Congress Classification Schedules

A comprehensive list of LOC Classification schedules.
http://www.loc.gov/catdir/cpso/lcco/lcco.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

Concept Space

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

The Astronomy Thesaurus

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

Thesaurus of Musical Instruments

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