Dictionaries

American Heritage dictionary of the English language 
Includes over 90,000 word entries. 
Cambridge dictionaries online 
Cambridge International dictionary of English, Cambridge dictionary of American English, Cambridge international dictionary of idioms, and International dictionary of phrasal verbs.
Dictionary.com 
A multi-source dictionary and thesaurus search service. 
Merriam Webster WWWebster Dictionary
On-line version based on the Merriam Webster collegiate dictionary, 10th edition and collegiate thesaurus.
OneLook
Searches 6,087,877 words in 967 dictionaries indexed.
Yourdictionary.com 
Maintains the most comprehensive and authoritative language portal on the web, with more than 2500 dictionaries. 

Thesaurus

Bartleby.com 
Features word definitions and a hyperlinked category index. Contains 35,000 synonyms and over 250,000 cross-references. 
Thesaurus.com 
Includes a brief definition, synonyms, antonyms and concept, the general idea underlying the terms in the entry. Arranged in dictionary style format, with over 17,000 entries 
Wordsmyth.net Original standard dictionary-thesaurus of American English, with definitions and pronunciations, including integrated thesaurus and usage examples. 
Onelook.com Search engine for words and phrases. More than 5 million words in more than 900