From the team that brought you Visual Studio Tools for Office--we announced a new product today called Visual Studio Tools for Applications.
Visual Studio Tools for Applications (VSTA) is a .NET-based application customization technology designed to be embedded into applications to enable developers to create custom experiences on top of those applications. It offers a full-featured interactive development environment based on Visual Studio techologies, design-time, runtime and supports both VB.NET and C#. ISVs can license the VSTA engine and integrate it into their applications so that their customers can customize their applications using .NET programming techniques. VSTA notably offers a significantly advanced environment for application customization and extensibility and is the result of over five years of effort and broad industry feedback. Office 12 is using VSTA for application customization in InfoPath. Also, we announced our first four external partners for VSTA at PDC.
Or to put it another way--if you have an application (.NET or native C++) with an object model and you want to license a Visual Studio based IDE that will let developers build extensions in VB.NET and C# on top of your application--VSTA is just what you are looking for.