Wikiwords, a project to create a comprehensive and open dictionary of all terms in all languages, has bee announced recently: a new project to create an open, multilingual lexicon of all terms in all languages.
Looks like it might become such a great site as wiki's encyclopedia.
It's still on Beta and the
homepage is here. (You'll have to create an account.)
It's even now, on the very first days of it existence, already a huge dictionary!