Happy Dictionary lets you quickly switch between several Internet dictionaries to look up terms in different places. Using Web Dictionary Links that you set up, you can quickly switch between multiple sites by pressing ⌘1 through ⌘0. Setting up Web Dictionary Links only simply requires you to take the URL of your desired site and replace the place {query} into the url where the search term should go.
One World Dictionary by Ascendo is a leading multilingual reference tool for Apple desktops.
InstantTranslate makes it really easy to translate a selection of text in the application you are working in from one language to another language.
