The Oxford English Dictionary (OED)
Provides the etymology, the origins and the historical development of the meaning of a word.

Merriam-Webster Online Dictionary & Thesaurus
Not sure what a word means or if you're using it correctly? Tired of using the same word over and over again? Find what you need here!

A comprehensive catalog of language-related Internet resources.

Internet Acronym and Abbreviation Server
Explains and defines acronyms.

One-Look Dictionaries
Search more than 595 general and specialized English dictionaries simultaneously, or select specific dictionaries.

Rhyming Dictionary
Type a word and find its rhymes, synonyms, definitions, and more.

Hosted by Dictionary.com, this is a great site to help diversify your word choice.

The Wordsmyth English Dictionary-Thesaurus
Find definitions, antonyms, pronunciations, sample sentences, and more. This site has three search levels, depending on the complexity of results you're looking for.

A comprehensive list of articles from more than 1500 dictionaries representing more than 230 languages.