• A Web of On-line Dictionaries at YourDictionary.com
  One-stop shopping for all things dictionary! This website indexes on-line dictionaries, thesauri, including over 800 dictionaries in 160 languages. "Preference in selection has been given to free online dictionaries of high quality." Specialized dictionaries on over 50 topics such as advertising, agriculture, architecture, biology, education, geology, law and medicine have recently been added.
• OneLook Dictionary
  "5,998,569 words in 964 dictionaries."
• OneLook Reverse Dictionary
  "OneLook's reverse dictionary lets you describe a concept and get back a list of words and phrases related to that concept. Your description can be a few words, a sentence, a question, or even just a single word."
• Allwords.com
  "English dictionary with multilingual search."
• Acronym Finder
  "Look up 128,000+ acronyms/abbreviations & their meanings. A searchable database containing common acronyms and abbreviations about all subjects, with a focus on computers, technology, telecommunications, and the military."
• American Heritage Dictionary of the English Language
  "Over 90,000 entries feature 10,000 new words and senses, 70,000 audio word pronunciations, 900 full-page color illustrations, language notes and word-root appendixes."
• Classical Dictionary
  From the Beazley Archive at Oxford, this dictionary explains terms from classical mythology and art. Heavy use of illustrations.
• Dictionary.com
• BibleStudyTools.com Bible Dictionary
• Online Etymology Dictionary
• Etymology of First Names
• Findlaw Legal Dictionary
• Foreign Language for Travelers
• Glossary of Art Terms from The Museum of Modern Art
• HyperDictionary
• Internet Glossary
  At University of California - Berkeley
• Lectric Law Lexicon
  Legal dictionary and reference room.
• Merriam-Webster's Dictionary
• Mondofacto
  Online Medical Dictionary
• PlantFacts
  Ohio State University Glossary of Horitcultural Terms
• Roget's International Thesaurus of English Words and Phrases
  "Peter Roget’s classic structure coupled with Mawson’s modernization becomes even more user-friendly on Bartleby.com with 85,000 hyperlinked cross-references—thus fulfilling their goal of “each word being related to its neighbors and each part to the whole.” Additionally, over 2,900 proverbs and quotations from classic and modern authors illustrate the 1,000-plus entries."
• Semantic Rhyming Dictionary
  Insert a word, your choice of perfect rhyme, last sound only, homophones, or synonyms.
• Standard Industrial Classification Scheme
• Symbols.Com
• TechWeb Technology Encyclopedia
• Whatis?com
  A dictionary of information technology, Internet, and computer terms
• The Word Detective
  "The Word Detective on the Web is the online version of The Word Detective, a newspaper column answering readers' questions about words and language. The Word Detective is written by Evan Morris and appears in finer newspapers in the U.S., Mexico and Japan."
• Word Origins
  "The words and phrases are selected because their origins are inherently interesting or some bit of folklore, sometimes true and sometimes false, is associated with the origin."
• World Wide Words
  This extensive site provides information on the origins of numerous words and phrases.
• The Wordsmyth English Dictionary-Thesaurus
  If you're not sure about spelling, use the Approximate Search and the computer will look for near matches ("suphose" retrieved "suffice, suffuse & suppose"); if you want to see all words that INCLUDE your word, do a Floating Search ("hand" retrieved glad hand, hand-to-hand, deck hand & hand grenade). This is also a dictionary of synonyms.

(updated 10/8/2015)