Webapp Translate is a highly crafted web wrapper app for Google Translate. It lets you select some text in any application and use the contextual menu to translate the selected text. The application can translate into 70 languages and can be accessed from the Dock or the Menu Bar.
InstantTranslate makes it really easy to translate a selection of text in the application you are working in from one language to another language.
One World Dictionary by Ascendo is a leading multilingual reference tool for Apple desktops.
