Visual Studio Tools for Office
Visual Studio Tools for Office (VSTO) 2.0 is now in beta!
Of course VSTO 1.0 has been out for some time now and is available here.
What's great about VSTO is that it allows developers a way to extend Office applications with managed code using Visual Studio .Net.
VSTO 2.0 works with Visual Studio 2005 and adds some powerful new capabilities such as hosting Word and Excel as full fledged designers in the VS environment, and creating powerful WinForms additions to your Office applications.
There is also a Visual Studio Tools for InfoPath Preview available here.