English Language Dictionaries
Dictionary.com: Searchable and free online dictionary and thesaurus.
Merriam-Webster: Free online dictionary and thesaurus with subscription access to an unabridged dictionary.
Cambridge Dictionaries Online: Free online dictionaries, includes a dictionary for English learners, a standard English dictionary, an idioms dictionary, and a phrasal verbs dictionary.
OneLook: Free dictionary that includes the ability to search with wild cards in place of some letters.
Your Dictionary: Free dictionaries that include definitions, synonyms, and usage examples.
Word Web Online: An international dictionary that includes American, British, Canadian, and Asian English spellings and words.
Newfoundland English: Free, searchable dictionary of words and phrases peculiar to Newfoundland and the Canadian coast. Includes definition and history for all entries.
Digital Dictionaries of South Asia: A searchable dictionary of Anglo-Indian words. Requires a Unicode font: instructions for obtaining one are included.
Yorkshire Dictionary: An alphabetical glossary of terms from the Yorkshire dialect of English.
Technical English Dictionaries
SNIA: Technical dictionary of terms related to storage networking and data and information management.
Tech Terms: Glossary style dictionary of computer and technology terms organized into several large categories and with the search features.
Webopedia: Searchable database of computer and Internet terms.
Acronym Finder: Searchable database of over 750,000 acronyms from many different areas of society.
NetLingo: List of acronyms used for text messaging and instant messaging.
All Acronyms: Comprehensive collection of acronyms divided by category and letter with the search feature.
Visual Thesaurus: Interactive thesaurus that displays words through a visually represented web of related words. Limited free trial after which a subscription is required.
Wordsmith: Searchable thesaurus with antonyms in addition to synonyms.
Encyclopedia Britannica: Online version of the Encyclopedia Britannica that requires a membership subscription to access most of the articles.
Enotes: A set of browsable encyclopedias divided up by subjects, including history, business, science, and science, and religion.
Encyclopedia.com: Online encyclopedia featuring information from published reference volumes. Subscription is required to access most content.
Infoplease: Free online access to articles from the Columbia Encyclopedia, Sixth Edition.
World Book Encyclopedia: Online version of the World Book Encyclopedia, requires a subscription to access.
VoiceNation: a live answering service that can be used on mobile phones with text messaging.
Word Reference: Translation and definition dictionary, includes English, French, Spanish, Italian, German, and Russian.
Beolingus: German dictionary that includes German-English, German-Spanish, and German-Portuguese translations.
The Century Dictionary: An online version of a 100-year-old dictionary, useful for looking up historic words. Requires the free DjVu plug-in to search and view.
Dictionary of Americanisms: An online version of a dictionary of American colloquialisms from 1848.
Rhyme Zone: Dictionary that allows searches for definitions, synonyms, antonyms, rhymes, and homonyms.
One Look Reverse Dictionary: Search for words by a concept or definition.