The American Heritage Dictionary of the English Language
Online publication of the 4th edition. Featuring audio word pronunciations, illustrations.

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 Rap Dictionary
Hip-hop slang and also contains a list of hip-hop artists and their real names.

The English-to-American Dictionary
A humo(u)rous resource for Americans learning British/Scottish English.

Ultralingua Online Dictionary
Searchable dictionary of the English language, with standard, reverse and phonetic lookup. Also links to a range of other language resources.

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 Beet Foundation
General purpose online English language dictionary and Thesaurus.

DataSegment.com: Online Dictionary
Find definitions by alphabetical browsing or word lookup. Powered by several print dictionaries.

Bibliomania: Webster's Unabridged Dictionary of 1913
Forms version. Allows browsing headwords.

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.

Dictionary.com
Online English dictionary, thesaurus and reference guide. Includes word games, online translator, and word of the day feature.

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.

AnsMe.com Dictionary
Finds definitions, similar-sounding words, relations, words that rhyme, and translations.

SmartDictionary.com
Supports both searching and browsing and encourages user contributions.

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.

dictionary.net
Comprehensive dictionary database of word and phrase definitions from 8 different sources.

Macmillan English Dictionaries
Resource site for the Macmillan range of dictionaries.