VSIP SDK now free
If you integrate or plan to integrate within Visual Studio, you should definitlty check this site.
"Register to become a Visual Studio Industry partner and get FREE access to the VSIP SDK. As a VSIP you will have access to the technology, private newsgroups and secured technical content to assist you in seamlessly integrating your tools, components, and languages into the Visual Studio .NET IDE."
Everything you always wanted to do to integrate seamlessly into VS.NET used to cost money. As of todays, it's free! Wahoo!
As Harry Pierson says, you could start with a look at the technical info.