I’ve found this really useful Merriam-Webster online dictionary. I use it all the time to check my spelling as it usually shows a list of alternatives and suggestions. Their thesaurus is quite good too offering antonyms as well as synonyms.

Dictionary.com isn’t as accurate or descriptive but it does offer a nerdy word-of-the-day by email service.

Babelfish offers translations for web pages and short text between quite a few European languages as well as Chinese.