A Concise Dictionary of Middle English: From A.D. 1150 to 1580. By A.L. Mayhew and Walter W. Skeat. In plain text, or as a zip file, from Project Gutenberg.
An Internet Dictionary.: Online dictionary lets you search for definitions, cross-references English dictionaries based on multiple databases. Cross-references English dictionaries based on multiple databases. Downloadable software gives the ability to integrate the dictionary with your browser.
AnsMe.com Dictionary: Finds definitions, similar-sounding words, relations, words that rhyme, and translations.
AskOxford.com: For questions on English grammar, spelling, and usage; to search a dictionary of words, names or quotations; for help with writing; for crosswords and puzzles.
Collins Word Exchange: Dictionary searches a variety of word and phrase resources, including the Collins English Dictionary, texting abbreviations, internet links, and words submitted by users.
DataSegment.com: Online Dictionary: Find definitions by alphabetical browsing or word lookup. Powered by several print dictionaries.
Dictionary of Indian English: Regionalisms defined.
Dictionary.com: Online English dictionary, thesaurus and reference guide. Includes word games, online translator, and word of the day feature.
Dictionary.net: Comprehensive dictionary database of word and phrase definitions from 8 different sources.
Die.net Online Dictionary: Cross-referenced definitions, spelling correction, and searches from WordNet, Webster's, and a variety of specialized sources. Supports regular expressions and the DICT protocol.
Elizabethan Dictionary: Provides the origins and meanings of Elizabethan words and vocabulary including obscure Shakespearean expressions.
English-dictionary.us: Fast no-frills English dictionary.
Freesearch Dictionary: A British English dictionary powered by the Cambridge University Press that contains around 150,000 words. The site offers a search feature or can be browsed by letter.
HyperDic: Based on the WordNet lexical database and thesaurus, this dictionary covers only a selection of the most frequent English words. Includes synonyms, antonyms, examples and pronunciation, and links between related words.
Longman Web Dictionary: Searchable dictionary of contemporary english
Macmillan English Dictionaries: Resource site for the Macmillan range of dictionaries.
Plan of an English Dictionary (1747): Letter written by Samuel Johnson to the Earl of Chesterfield setting out his plan for an English Dictionary.
SmartDictionary.com: Supports both searching and browsing and encourages user contributions.
Suggestive Dictionary: An english dictionary which suggests words as you type. Helps explore new words and meanings.
The American Heritage Dictionary of the English Language: Online publication of the 4th edition. Featuring audio word pronunciations, illustrations.
The Beet Foundation: General purpose online English language dictionary and Thesaurus.
The English-to-American Dictionary: A humo(u)rous resource for Americans learning British/Scottish English.
The Jargon Database: A repository of popular neologisms, management speak, and technical terms relevant to various regions, jobs, and cultures, and organized by categories.
The Wordsworth Compendium: Program containing over 516,000 English words and phrases to solve word puzzles. Useful for the search and manipulation of words in all aspects of logological activities.
Ultralingua Online Dictionary: Searchable dictionary of the English language, with standard, reverse and phonetic lookup. Also links to a range of other language resources.
Vdictionary.com: Searchable English language dictionary shows results in synonym sets.
Webster's Unabridged Dictionary of 1913: Forms version. Allows browsing headwords.
Word Sytes: Use this online dictionary to search for definitions in WordNet, Webster's and many other databases.
WordIQ Dictionary: Searchable definitions available in a variety of dictionaries including Webster's, a legal dictionary, a dream dictionary, and an encyclopedia.