• Roget's Thesaurus   
  
  Look up words and find similar meanings.
  http://www.thesaurus.com/
  
   Enter a word:
   

• ARTFL Project: ROGET'S Thesaurus Search Form
  
  From the University of Chicago.
  http://humanities.uchicago.edu/orgs/ARTFL/forms_unrest/ROGET.html
• Astronomy Thesaurus
  
  From the International Astronomical Union.
  http://msowww.anu.edu.au/library/thesaurus/english/
• INGRID's Thesaurus
  
  Here you can see present subjects, look for genus subjects (hypernyms) and narrower subjects (hyponyms) of given subject, and of course, you can find records with given subject.
  http://ingrid.utlib.ee/thesaurus.html
• Lexical FreeNet
  
  This program allows you to search for relationships between words and concepts that might never have occurred to you before.
  http://www.lexfn.com/
• Thesaurus of Geographic Names
  
  An automated source of hierarchically arranged geographic data that is global in scope. From the Getty Information Institute.
  http://www.getty.edu/research/conducting_research/vocabularies/tgn/
• Web Thesaurus Compendium
  
  Index of other thesauri found on the Internet.
  http://www.ipsi.fraunhofer.de/~lutes/thesoecd.html
• WordNet
  
  An online lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept.
  http://wordnet.princeton.edu/
• Webster's Thesaurus
  
  From Merriam-Webster.
  http://www.m-w.com/thesaurus.htm
• Wordsmyth Dictionary-Thesaurus
  
  Integrated dictionary and thesaurus.
  http://www.wordsmyth.net/

     See also, these cross-references:

• Acronyms and Abbreviations
  
• Almanacs and Facts
  
• Dictionaries
  
• Encyclopedias
  
• Famous and Familiar Quotations
  
• Translations