Google Translate is one of their newest services, and it offers the ability to translate words from one language to another as you type, in real time.  If you upload a document, it will translate the entire text to the language of your choice.  It also has a speech feature that helps you determine how to say a word with just a click of the mouse.

I can't necessarily see how I would use this very often, but the usability of it is really cool.  The video below is a tutorial from Google on how the tool works.