The Century Dictionary Online - Free 500,000 word dictionary of English with an encyclopedia and world atlas. Uses DjVu and JPEG formats to display page images. Has headword lookup and full text search of 22 million words of underlying text.
Dictionary Society of North America - An international organization of librarians, booksellers, translators, linguists, publishers, writers, journalists, and collectors. Founded in 1975 to further research in dictionary making, study, collection, and use. Member of the American Council of Learned Societies.
Dictionary.com - Free online dictionary search, translator, word of the day, crossword puzzles and word games, and vocabulary learning resources for many languages.
Dictsearch - Search a variety of dictionaries including monolingual, bilingual, and specialty dictionaries.
Voycabulary - Online dictionary and thesaurus web linking lookup reference tool.
Web Dictionaries - Multi-search tool by Tinh Van, which consults English-only dictionaries, and Vietnamese into English or French translation dictionaries.
Webster's Online Dictionary: The Rosetta Edition - This database doubled with a metasearch tool extracts records from a variety of online and public-domain paper dictionaries, both general and specialized. It offers English definitions, as well as translations from/to a hundred languages.
Word Assault - Looks up multiple words simultaneously using several databases including Webster's, Easton's, Wordnet, and the Devil's Dictionary.
Word Central - Student dictionary and rhyming dictionary online at Merriam-Webster's website. Daily Buzzword by subscription.
Word Spy - Site dedicated to recently coined words and existing words revived into modern usage.
WordWeb - A free English thesaurus and dictionary for Windows.
Worthless Word for the Day - Obscure, abstruse and/or recondite word, especially one not falling into medical terms, foreign monetary units, foreign units of measure, legal terms, or professional jargon of any type.
yourDictionary.com - Comprehensive index of on-line dictionaries in more than 200 different languages. Includes an index of on-line grammars, word of the day by email, and several pages of linguistic fun.