Dictionaries
AlphaDictionary - http://www.alphadictionary.com
A free online English dictionary, dictionaries and grammars for 350 languages.
Cambridge Dictionaries Online - http://dictionary.cambridge.org/
Publishes a selection of English dictionaries for learners worldwide. Also offers tools, resources, word of the day and blog.
The CMU Pronouncing Dictionary - http://www.speech.cs.cmu.edu/cgi-bin/cmudict
North American English pronunciation site. Pronunciations are machine-read and has over 100,000 words in the database.
Thesaurus.com - http://thesaurus.com/
Online thesaurus and dictionary site. Also has a quotes section.
Word Central - http://www.wordcentral.com/
Provides a dictionary and rhyming slang dictionary for students with extra features on subscription.