Dictionaries & Thesauri

Dictionaries

• Dictionary.com-Definitions from Webster's and more.
• OneLook Dictionaries - searches more than one dictionary per query.
• Wiktionary
• YourDictionary.com:

Specialized Dictionaries

• Online Medical and Science Dictionaries
• Pseudodictionary
• Rhyming Dictionary
• Urban Dictionary

Thesauri

• Roget's Thesaurus
  
• Visual Thesaurus
• Mathematical Thesaurus
  

Word Reference

• Acronym Finder
• Memidex (free online dictionary and thesaurus)
• The Word Spy (word lover's guide to new words)