For queries about the English language in general. The site is devoted to good English. Has an Ask the Experts section, daily and weekly quote.
You can type in a word to find its definition and you can also order online from a list of useful dictionaries.
Dictionary on the Encyclopaedia Britannica site. Type in your word to see definition and pronunciation.
Oxford English Dictionary Online. You have to subscribe to OED to use the service which is updated quarterly with at least 1000 new and revised entries