Merriam-Webster Dictionary  - official site for the reference publisher. Includes online versions of the collegiate dictionary and thesaurus.

OneLook Dictionaries  - search for multiple definitions of a word from a variety of online dictionaries.

allwords.com - English dictionary with multi-lingual search capability. Includes crossword solver features.

American Heritage Dictionary of the English Language, Fourth Edition - features over 90,000 entries, most with audio word pronunciations.

AskOxford.com  - contains a dictionary and thesaurus, as well as quotations, word of the day, games, and lexicography.

Cambridge International Dictionaries - search separate dictionaries of English, phrasal verbs, or idioms.

Century Dictionary Online - American dictionary with fulltext search page images representing the definitions, atlas, and cyclopedia from the original text.

CoolDictionary.Com: Audible Webster Dictionary - features audio clips of words and quotes.

DICT Development Group  - simultaneously search several dictionaries for word definitions including Websters, Jargon File, and other databases.

Dictionary of American Regional English - specializes in regional words and phrases.

Dictionary.com - features a variety of tools including thesauri, translator, and writing resources.

Encarta World English Dictionary - features audio clips of words pronounced.

FreeDictionary.org - simultaneously and efficiently searches several dictionaries for word definitions.

Google Glossary - find definitions, phrases, and acronyms.

HyperDictionary - displays definitions with each individual word in the definition hyperlinked to its definition.

Hypertext Webster Gateway - offers point-and-click interface for accessing various dictionary services on the Internet.

iTools Language Tools - features dictionary, thesaurus, and translation tools.

LookWAYup - dictionary and search tool embedded into the web page.

VoyCabulary - makes the words on any webpage into links so you can look them up in a dictionary or other reference site, by simply clicking on the words.

WebRef.org - providing glossaries of terms related to various scientific disciplines, aesthetic arts, and biographical information.

Word Assault - searches various dictionaries for multiple words at a time.

WordNet - online dictionary that finds words that mean the same, are more general, or more specific.

Wordsmyth Educational Dictionary-Thesaurus - American English dictionary with an integrated thesaurus and other lexical resources.

Yahoo! Reference - provides dictionary, encyclopedia, quotation, and world fact resources.