Wiktionary is a collaborative project to produce a free-content multilingual dictionary in many languages. The project has grown beyond a standard dictionary and now includes a thesaurus, a rhyme guide, phrase books, language statistics and extensive appendices.
You can choose among quite a few languages for the interface, and within one language (such as English) you can search for a word in many languages. The word will be defined in the language that you have chosen as an interface.
Wiktionary is available at http://en.wiktionary.org/wiki/Wiktionary:Main_Page