Synonymous.com

A Concise Dictionary of Middle English
From A.D. 1150 to 1580. By A.L. Mayhew and Walter W. Skeat. In plain text, or as a zip file, from Project Gutenberg.
www.gutenberg.org/

English-dictionary.us
Fast no-frills English dictionary.
www.english-dictionary.us/

The English-to-American Dictionary
A humo(u)rous resource for Americans learning British/Scottish English.
english2american.com/

Internet Picture Dictionary
With interactive activities for French, German, Spanish, Italian, and ESL learners.
www.pdictionary.com/

Review of T. Crowley (1990) Bislama-English and English-Bislama Dictionary
As published in Oceanic Linguistics, volume 30, 1991.
www.uhpress.hawaii.edu/

Online English to Japanese to English Dictionary
JavaScript EDICT server from Freedict.com with romanized Japanese to English and English to romanized Japanese. (GIF)
www.freedict.com/

'Strine Decoded: Dictionary of Australian English
Dictionary covers Australian English, including slang, rhyming slang, Aboriginal English, place names, and animals.
www.artistwd.com/

Plan of an English Dictionary (1747)
Letter written by Samuel Johnson to the Earl of Chesterfield setting out his plan for an English Dictionary.
andromeda.rutgers.edu/

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

Longman Web Dictionary
Searchable dictionary of contemporary english
www.ldoceonline.com/