I was a little surprised to hear that some of the ISVs I work with had never heard of VSTA. Visual Studio Tools for Applications (VSTA) is a platform for Independent Software Vendors (ISVs) to add managed application customization to their applications. Notably, the VSTA team recently launched their VSTA 2005 SDK. Check out the VSTA team blog where they have posted a video to take you through the features and design goals of VSTA.
VSTA will have a big impact for ISVs that allow systems integrators or end users automate or customize their application or develop add-ins for their applications.
Blurb from the SDK docs; "With the familiar Visual Studio development environment, VSTA enables Visual Basic and Visual C# developers to easily build managed customizations for both COM and .NET applications. Sharing a common toolset with Visual Studio, VSTA provides a wealth of design-time features, a robust programming model, and seamless promotion of VSTA projects to full Visual Studio."