Collins English Thesaurus 1.03

The Collins dictionary, produced by Ultralingua and HarperCollins, includes easy to use interfaces, extensive dictionary databases, and seamless interaction with other applications. It boasts over 80,000 carefully edited entries, including phonetic transcriptions, drawing on a 2.5 billion word database.

Just click on any text in nearly any desktop application and a small pop-up window appears with the translation or definition. It includes inflected forms, recognizing conjugated verbs, etc. You can add other language modules and even create your own entries.