SpellCheck.net - A spellchecker plus a handy array of math and language links. Use KitchenMath to convert pints to cups or try Babel Fish to translate Spanish to English.
Google Translate - translates words or phrases both to and from English, French, Spanish, German, Italian, and Portuguese. This site will also translate entire web pages if you enter the URL.
The OED is considered by many to be the definitive record of the English language, featuring 600000 words, 3 million quotations, and over 1000 years of English.