Here are the resources mentioned in my MSDN Webcast: VSTO Straight from the Experts on May 9th, 2007.
The Guided Tour for VSTO Excel that will leap-frog you over the VSTO learning curve:
VSTO 2005 Guided Tour.doc
VSTO Excel Guided Tour Source
The Sample applications / reference applications that will get you rolling on building your own applications:
This application can be run in any version of VSTO. it has no design time surface. it's just code that mines a SQL server database, sorts, pretties it, creates a pivot table and then charts the data. you can easily modify this (and many have) to point at your own database
Great Plains or AdventureWorks Sales Orders Source
And here is an awesome VSTO 2005 Excel application I built that facilitates computer management remotely with WMI through VSTO. What's totally cool about this application is that it does an amazing amount of functionality with very little code. This is something you'd never do - manage the computers on your network with Excel - but, the simple fact that you can do it; and do it pretty easily, is a testament to VSTO's power and developer productivity.
VSTO Excel Win32 Services Source