This site is notable for allowing you to search the 1913 Merriam-Webster, that is, W1 revised. It is called dict.org because it supports the Dictionary Server Protocol (DICT), which is a general 1997 Internet protocol (RFC2229) for online dictionaries.
Free online English dictionary, thesaurus and reference guide, crossword puzzles and other word games, online translator and Word of the Day.
"Double-Tongued Word Wrester records undocumented or under-documented words from the fringes of English. It focuses upon slang, jargon, and other niche categories which include new, foreign, hybrid, archaic, obsolete, and rare words. Special attention is paid to the lending and borrowing of words between the various Englishes and other languages, even where a word is not a fully naturalized citizen in its new language."
Contains an English dictionary and several specialized ones. The English dictionary is based on Princeton University's WordNet. It contains around 150 thousand terms with examples, synonyms, antonyms, and related words. Can be searched or, more useful for our purposes, browsed.
A free online dictionary of weird and unusual words to help enhance your vocabulary. The IHL is a component of The Phrontistery, which has many other free word lists and unusual word related resources. Link contributed by Judy Madnick.
Merriam-Webster provides a free online dictionary, thesaurus, audio pronunciations, Word of the Day, word games, and other English language resources. You have to pay for a premium service to get full access to W3.
The basic sources of this work are Weekley's An Etymological Dictionary of Modern English, Klein's A Comprehensive Etymological Dictionary of the English Language, the Shorter Oxford (second edition), Barnhart Dictionary of Etymology, Kluge's Etymologisches W?rterbuch der Englischen Sprache, Ayto's 20th Century Words, and Chapman's Dictionary of American Slang.
Searchable online version of Roget's New Millennium? Thesaurus, First Edition. Just as every American dictionary can call itself Webster, so every synonym dictionary can call itself Roget. This one seems to be arranged alphabetically, rather than by meaning, and so it is not a thesaurus in Roget's sense at all, but a dictionary of synonyms.
Wordnik is an online dictionary founded by Erin McKean, former Editor-in-Chief for American Dictionaries at Oxford University Press. It differs from almost every other online dictionary in that it is conceived as a mainstream dictionary that was online from the ground up: not the online presence of a printed dictionary, nor a website about words, nor a blog. And its lexicographical respectability is top-drawer.