Just a few thesauri out there in use. If you have anymore to add, just drop me an email.

Web Thesauri Compendium

Alcohol and Other Drug

Art and Architecture Thesaurus

ASIS Thesaurus of Information Science

Getty Geographic Names

MeSH

NAL Agricultural Thesaurus

Bloom - Taxonomy of Education Objectives
DOT - Dictionary of Occupational Titles
WICH - Management Information Systems Program
ERIC - Education Resources Information Center Thesaurus
WordNet
Cyc - Cyc Ontology
HS - Harmonized Commodity Description and Coding System