Listings of Thesauri, Taxonomies and Classification Systems

Classification Schemes and Thesauri Online
This thesaurus collection is updated irregularly by the Laboratory for Bibliographic Information Retrieval (BIR). It includes both English and non-English language vocabularies as well as multilingual thesauri.
http://www.fbi.fh-koeln.de/institut/labor/Bir/thesauri_new/thesen.htm

Controlled Vocabularies, Thesauri and Classification Systems Available on the WWW
Compiled by Traugott Koch, this listing includes thesauri, dictionaries and classification schemes that are available on the Internet.
http://www.lub.lu.se/metadata/subject-help

Controlled Vocabulary.com
Includes a brief description of sample controlled vocabularies and thesauri available on the Internet.
http://www.controlledvocabulary.com/examples.html

DB Reference Thesaurus Page
Extensive list of thesauri and thesaurus Web sites, plus links to extensive reference materials on all variety of subject matter.
http://www.dbreference.com/thesaurus

Queensland University of Technology
Thesaurus online sites
http://sky.fit.qut.edu.au/~middletm/cont_voc.html#Thesauri

Classification schemes online
http://sky.fit.qut.edu.au/~middletm/cont_voc.html#Classification

Thesauri and controlled vocabularies
While the primary focus is on Canadian government vocabulary, listings of non-government thesauri are also included.
http://www.collectionscanada.ca/8/4/r4-280-e.html

UN Classifications Registry
The Classifications Registry provides updated information on statistical classifications maintained by the United Nations Statistics Division (UNSD).  Classifications cover products and activities and are international in language and scope.
http://unstats.un.org/unsd/cr/registry/

Web Thesaurus Compendium
A listing of thesauri and other structured vocabularies available on the Web compiled by Barbara Lutes; last modified in October 1998.
http://www.ipsi.fraunhofer.de/~lutes/thesoecd.html

Zthes Thesauri on the Internet
Listing of Zthes-compliant thesauri available on the Internet.
http://zthes.z3950.org/thes/index.html