Online Dictionaries  

 Dictionary.com 

Dictionary.com gives definitions from several different dictionaries on the results page. Thesaurus looks up Roget's Thesaurus.

• Select either dictionary or thesaurus

 OneLook 

OneLook gets results from hundreds of dictionaries, but requires an extra step to each definition. Best for serious research or unusual / topic specific words ( medical terms, foreign language etc.)

• HINT: Adding wild cards expands the search. (* = any number of characters and ? = exactly one character). For example, "tomo*ow", "tomo??ow" or "tomor*" will find "tomorrow".
• Includes 41 sports, 16 religious, 54 medical, 57 computer & 61 financial dictionaries!
• Try their new "reverse dictionary" - enter a meaning and find the words

 Merriam-Webster 

MW is an old, reliable American resource...

• Click on the audio symbol to hear how the word is pronounced - if this doesn't work, try the "Click here to listen with your default audio player." link in the popup box.
• There'll also be a set of pronunciation symbols below the search results
  

 WordSmyth 

Definitions from a single source - but is better looking, smarter and easier to use than those above

• Free registration is required
• It is a dictionary and thesaurus combined
• Home of the best advanced search I have seen
  

 Rhymes 

Rhyme Zone (part of Lycos) - great for poets!

 Visual Thesaurus 

"Through its dynamic interface, the Plumb Design Visual Thesaurus alters our relationship with language, creating poetry through user action, dynamic typography, and design. The experience is rewarding from both a linguistic and an aesthetic perspective."

"The Visual Thesaurus accesses data from the WordNet database, a publicly available resource developed by the Cognitive Science Laboratory at Princeton University. This database, first created in 1985 as a dictionary based on psycholinguistic theories, contains over 50,000 words and 40,000 phrases collected into more than 70,000 sense meanings."

 More Resources 

• VoyCabulary - makes the words on any webpage into links so you can look them up in a dictionary, by simply clicking on the words.
• What Is? - Definitions for thousands of the most current IT-related words
• WordSpy Top 100 - lots of newly coined words and phrases - see also Urban Dictionary
• Phrase Finder - type in a word and it returns a list of phrases that are related to your word in some way - not free!
• Quotations & Proverbs - search many engines at Fagan Finder
• Your Dictionary - more than 1800 dictionaries, in over 250 languages - the place to find specialty dictionaries
• Cambridge Dictionaires Online - search the famous dictionary, or see what others mostly search for
• Oxford FAQ - a database of some of the questions sent into the Oxford Word and Language Service team. You can ask them question yourself, if you want.
• Psuedo Dictionary - 14,000 made up words
• Strange & Unusual Dictionaries - covers one letter words, all-consonant and all-vowel words
• Devil's Dictionary - by Ambrose Bierce, 1911
• Grandiloquent Dictionary - over a thousand of the most obscure and unusual English words
• Symbols - more than 2,500 Western signs, arranged into 54 groups according to their graphic characteristics.
• Acronym Finder - over 3 million acronyms and abbreviations listed / Abbreviations - 348,000 acronyms & abbreviations
• Spell Check - just enter some text and get it spell-checked like Word does but online, with a letter and word count as well
• Oxford Dictionary CD-ROM - this place has the cheapest copies ($198 last time I looked)

ORIGIN OF WORDS

• Take Our Word For It - weekly word-origin webzine
• World Wide Words - the interesting history behind over a thousand of words and phrases
• Online Etymology Dictionary - probably the largest free online etymological database
• Origin of English Sayings - 500 sayings, and the origin of most

PUZZLE TOOLS

• Jumble and Crossword Solver / One Across - both handy for solving word puzzles
• Wordsmith - Anagrams - stick your name in and press the button!