Visual Studio eXtensibility (VSX) is the ability to develop extensions to the Visual Studio IDE from Microsoft. Extensions are developed using the VSSDK.
Links:
- Getting Started with Extending Visual Studio
- The Visual Studio SDK for 2010
- The VS2010 beta forums
- Visual Studio 2010 New Features, Extensibility Points and Partner Opportunities
- Visual Studio 10 extensibility samples
Most of the information on add-ins is in the SDK.