Visual Studio gives you a lot of extensibility options. You can create your own SDKs, use the Windows, Windows Phone, and Azure SDKs (which are installed as part of Visual Studio), and extend Visual Studio itself.
Extend Visual Studio
You can use the Visual Studio SDK to extend just about every part of Visual Studio: commands, menus, windows, editors, and projects. To find out more, see Visual Studio SDK.
Create Your Own SDKs
Find out how to create, package, and deploy your own platform and extension SDKs: Creating a Software Development Kit.
Find out how to use Roslyn for extensibility: .NET Compiler Platform ("Roslyn") Extensibility.