Common Errors in English - Catalogue of mistakes and easily confusable words.
Merriam Webster's Dictionary and Thesaurus
OneLook Dictionary - Search more than 130 dictionaries at once.
Rhyming Dictionary - Enter a word and find a corresponding list of words which rhyme with it.
Visuwords - Visuwords turns the dictionary into a neural network; it's the perfect interface for writers, readers, and anyone who's curious about the English language.
William Strunk Jr's The Element of Style - Online publication of Strunk's classic guide to the English language.
YourDictionary.com - Find definitions in English, German, French, Greek, Latin and other languages.