1)   Add-in Express .NET for VSTO 2007.3.4
Easily develop professional extensions for Microsoft Office. Build Office add-ins with minimal coding and debugging, customize Office 2007 Ribbon UI and Office 2003 toolbars with special visual designers in your VSTO 2005 (SE) projects.

 2)   Add-in Express Toys for VSTO and Excel 2.0
This is a free sample add-in (ready to install and use) that adds some useful features to Microsoft Excel. It contains a full source code in VB and C# and supports Excel 2003 and 2007. The add-in works in VSTO 2005 SE and VSTO 2008 (VB.NET, C#).