American Heritage Dictionary of the English Language (Bartleby)
Provides over 90,000 entries, featuring 10,000 new words and 70,000 audio word pronunciations.

Cambridge Dictionaries Online
Provides definitions to 700 million words used in written and spoken English.

General Dictionaries 
Dictionaries offered by Credo Reference.

Merriam-Webster Online
An online dictionary with links to word games and kid's pages.

Oxford English Dictionary 
The OED Online contains the complete text of the 20-volume Third Edition. This dictionary contains in depth information about each word or term, including etymology and historical examples.

Webster's Revised Unabridged Dictionary, 1913 ed.
Roget's Thesaurus and Webster's dictionary are available at this site developed by the American and French Research on the Treasury of the French Language (ARTFL).

Wordsmyth English Dictionary-Thesaurus
Offers clarity, simplicity, and precision of style which results in definitions that are more accessible than those of American college dictionaries. Dictionary and thesaurus data are integrated so that only one entry is required instead of both dictionary and thesaurus entries.

Online Slang Dictionary
A global dictionary of current and antiquated slang terms.

Dictionary of Contemporary Slang 

Bloomsbury Thesaurus 

Merriam-Webster Thesaurus
Use the thesaurus to avoid repeating words within a sentence and avoid beginning successive sentences or paragraphs with identical words.

Roget's II: The New Thesaurus 

Thesaurus.com
Find synonyms and increase your vocabulary by using a thesaurus.

Art Dictionaries 

Bilingual Dictionaries 

Biographical Dictionaries 

Business Dictionaries 

Geographic Dictionaries 

Historic Dictionaries 

Linguistic Dictionaries 

Law Dictionaries 

FindLaw
A law dictionary and resource for finding legal help.

Literature Dictionaries 

Medical Dictionaries 

Online Medical Dictionary
It aims to provide a one-stop source of information about all medical and scientific terms and includes many useful cross-references and pointers to related resources elsewhere on the Internet, as well as bibliographical reference to paper publications.

Music Dictionaries 

Philosophy/Psychology Dictionaries 

Religious Dictionaries 

Science Dictionaries 

Social Science Dictionaries 

Technological Dictionaries