AllWords
Provides definitions, translates words into five different languages, and lets you listen to word pronunciation.

Cambridge International Dictionaries Online
Allows you to search four different online dictionaries, including the Cambridge International Dictionary of English, the Cambridge Dictionary of American English, Cambridge International Dictionary of Phrasal Verbs, and the Cambridge International Dictionary of Idioms.

The Newbury House Online Dictionary
"This online dictionary contains over 40,000 entries and is based on the best selling Newbury House Dictionary of American English."

Oxford English Dictionary
The online version of the seminal dictionary of the English language. Registration required.

Web of Online Dictionaries
Includes an index of hundreds of online dictionaries in a variety of languages. Subscription required.

Merriam-Webster Online
Large site by Merriam-Webster that features the famed dictionaries and reference products.

Thesaurus.com and Dictionary.com
Related sites that include links to writers' resources, games, word information, and more.

WordNet
Yet another "online lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory."

Wordsmyth
American English dictionary and thesaurus.