VSSDK Assist is a set of utilities and tools to help people who want to start extending Visual Studio. Pablo wrote this in his “spare time” while helping us ship Service Factory: Modeling Edition! He gave a talk to the p&p team about some of the things it lets you. In short it makes getting started with package development a snap.
There’s also really helpful article in CoDe Magazine to get you started, Learning the Visual Studio SDK with VSSDK Assist. Doug Hodges also mentions VSSDK Assist and p&p guidance packages in an interview about the history of the Visual Studio IDE in the same issue.
If you want to extend the Visual Studio IDE then this is definitely worth checking out.