From Add-in Express:
Use visual RAD designers, secure and isolated components, deployable and updatable templates of this component set to program professional extensions for Microsoft Office with minimal coding and debugging.
Enjoy lots of advanced features for VSTO and VSTO (SE): customize Office 2007 Ribbon UI and Office 2003 toolbars at design-time and run-time, add elements to Ribbon tabs, the Quick Access Toolbar and Office Menu and validate the resulting Ribbon XML in no time. Use special components to create custom task panes, application-level keyboard shortcuts and to embed custom .NET controls into Office toolbars. If you develop plug-ins for Outlook, get Outlook-specific command bars, option pages, and the ability to embed custom .NET forms into Outlook Explorer and Inspector windows, including the complete replacement of a folder view with a .NET form like Outlook Today does.
Add-in Express 2007 is based on the security and deployment model of VSTO 2005 and VSTO 2005 SE, developed in C# and delivered with its source code. It completely supports application-level add-ins for Outlook, Excel, Word, PowerPoint, Visio and InfoPath, and works for Visual Basic .NET and C#.