Microsoft has deprecated the Visual Studio Add-in infrastructure. According to MSDN, “Visual Studio add-ins are deprecated in Visual Studio 2013. You should upgrade your add-ins to VSPackage extensions.” The documentation later says that “VSPackages are the principal architectural unit of Visual Studio, and are the unit of deployment, licensing, and security also. Visual Studio itself is written mostly as a collection of VSPackages.”
Existing add-ins will continue to work for now, but new ones cannot be created using VS 2013. That said, developers should consider following the How to: Convert an Add-in to a VSPackage guide on MSDN as soon as possible. With the increased cadence of Visual Studio releases it may not be long before add-ins are removed completely.
VSPackages have been available for over a decade but were often avoided because they required Visual Studio Industry Partners (VSIP) license along with Developer and Package Load Keys. As of 2010 the MSDN documentation doesn’t mention this requirement. There was also license restriction prior to 2005 that said VSPackages couldn’t be open source.