Office Design Site
One of the things that is the most exciting about Office 2007 is the new user interface. Jensen Harris has been providing an ongoing commentary of the development of this new UI, including many screenshots and now movies of the user interface elements in action.
The Office Design site is another great way to get insights into the design thinking behind Office 2007. Both of these sites are worth tracking; I learn something new every week from them. I particularly like the insights behind why certain decisions were made and the demonstrations on Jensen's site for how the new features work.