D4G is a new dictionary addon for firefox. By default it uses several database sources on the web : Babylon, Google, Dictionary, Reference, Therasus etc.. to translate word or pharse. You can also add your own favorite website.
Trigger the translation by pointing mouse at the word and press Ctrl-Mouse Click, or selecting words or pharses and press Ctrl-Z
There is also pronouciation. It's extremly customable and useful.