Merriam-Webster
Look to the Merriam-Webster website for proper spellings, pronunciations and definitions of thousands of English words. You can also access their thesaurus and foreign-language dictionaries here.

Wordreference
A very user-friendly site that translates words from English into Spanish, German, French, or Italian with a click. Also translates foreign words and phrases into English - muy bueno!

Handspeak
Sponsored by Handspeak, this sign language dictionary is the "largest, fastest growing visual (sign) language dictionary online". Results show real-action demonstrations, not just still pictures!

Encyclopedia Britannica
The Encyclopedia Britannica online is a great place to start a search on just about any topic. The powerful site connects you to news articles, maps, photos, summaries and lots of useful information. If you want to browse ad-free, though, it'll cost ya: see "subscribe".