Merriam-Webster OnLine - A free, searchable on-line dictionary and thesaurus, word games, a word of the day, and many other English language and vocabulary reference tools and resources.
Bartleby.com - Online publication of The American Heritage Dictionary, Fourth Edition. Featuring 70,000 audio word pronunciations, 900 full-page illustrations and over 10,000 new words and senses.
AskOxford.com - For questions on English grammar, spelling, and usage; to search a dictionary of words, names or quotations; for help with writing; for crosswords and puzzles.
The Online Slang Dictionary - A global dictionary of English slang.
The Rap Dictionary - Hip-hop slang and also contains a list of hip-hop artists and their real names.
A Dictionary of Slang - English slang and colloquialisms currently used in the UK.
Bibliomania: Webster's Unabridged Dictionary of 1913 - Forms version. Allows browsing headwords.
The English-to-American Dictionary - A humo(u)rous resource for Americans learning British/Scottish English.
