Merriam-Webster OnLine: Free searchable dictionary and thesaurus, word games, a word of the day, and many other English language and vocabulary reference tools and resources

Oxford Dictionaries: Dictionary by Oxford University Press. Answers questions on English grammar, spelling, and usage. Basic dictionary is free, premium content with subscription

Urban Dictionary: A searchable archive of contemporary American slang, listed in alphabetical order. Allows its users to create definitions for slang words and phrases and submit them for publication. Includes a "Word of the Day" section, chat features and live minute-by-minute site update logs