TranslateIt! for Mac is a multilingual dictionary that allows you to look up words on the fly. Just hover the pointer over an unfamiliar word in any Cocoa Application, and you will see its translation in a pop-up window instantly.
TranslateIt! for Mac features: look up words in any applications by just moving the pointer over them. English, German, French, Spanish and Russian morphology support. Lots of free dictionaries for a great number of languages. Integrated converter for XDXF, Babylon, SDICT, Apple Dictionary, DSL, DCT formats. Translation from/to any language. Support for most widely known online dictionary/translation services - it allows TranslateIt! to directly query such services, retrieve results, and display them in a pop-up window, the main app window, or TI! Widget.
The program has a simple interface and is quite easy to use. Still it is rather costly.