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.
The Beet Foundation - General purpose online English language dictionary and Thesaurus.
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.
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.
Dictionary.com - Online English dictionary, thesaurus and reference guide. Includes word games, online translator, and word of the day feature.
Dictionary.net - A dictionary database of word and phrase definitions from eight 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.
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.
The Jargon Database - A repository of popular neologisms, management speak, and technical terms relevant to various regions, jobs, and cultures, and organized by categories.
New Dictionary Words - An interactive quiz to test you knowledge of new words addd to the Merriam-Webster dictionary.
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 one types. Helps explore new words and meanings.
Ultralingua - 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.
Wiktionary - English language Wiki dictionary from the Wikimedia Foundation.
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.
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.