Dictionaries and Thesauri

• Oxford English Dictionary (OED)
  historical/etymological dictionary of English
• Oxford Reference Online: Bilingual Dictionaries
• Oxford Reference Online: More Dictionaries
  modern English language
  
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• Merriam-Webster Online  
  Online version of the Merriam Webster Collegiate Dictionary
  A thesaurus is also available
• Lexicool.com
  Directory of bilingual/multilingual dictionaries on the web
• OneLook Dictionaries Searchable database that provides definitions and pronunciations for more than 4 million words in 800 dictionaries
• Online Language Dictionaries
  Compilation of links to dictionaries of languages used throughout the world
• Specialty Dictionaries
  Compilation of links to many subject specific dictionaries
• Thesaurus.com   
  Web version of Roget's Thesaurus
• YourDictionary.com  
  Includes definitions from more than 1800 dictionaries in over 250 languages.  Includes a thesaurus and translator