As with Vista, the Office 2007 system presents a fantastic new opportunity for software developers and ISVs. To make sure you are taking advantage of this check out the Office dev center over at MSDN which has lots of info on how you can code for the new platform.
Some of my favourites are:
Custom Task Panes
Customising the ribbon in Outlook 2007
The Groove 2007 SDK
Alternatively check out all the resources here:
ProgramsAccess, Excel, InfoPath, OneNote, Outlook, PowerPoint, Project, Visio, Word
ServersProject Server, SharePoint Server
ServicesGroove 2007
Tools and TechnologiesAdd-ins, .NET in Office, Ribbon, Visual Studio Tools for Office, Windows SharePoint Services, XML in Office