VS 2010 provides a much richer extensibility model than
previous releases. Anyone can build extensions that add, customize, and
light-up the Visual Studio 2010 IDE, Code Editors, Project System and
associated Designers.
VS 2010 Extensions can be created using the new MEF
(Managed Extensibility Framework) which is built-into .NET 4. You can
learn more about how to create VS 2010 extensions from this this blog post from the Visual Studio Team Blog.