When MS Office 2007 was released, you can't disagree with the fact that one of the biggest highlights was the User Interface! I went to a Office 2007 Beta Developer Deep Dive course and they taught us how to extend ribbons and the user interface, now on MSDN, they teach you the lot in detail! It's definitely awesome what you can do with these ribbons!
Go check it out:
* Extend The 2007 Office System With Your Own Ribbon Tabs And Controls
* Jensen Harris' Office Blog