As has already been mentioned by Chetan and Craig, the Automation (EnvDTE) Samples for Visual Studio 2005 have been released. If you're interested in seeing what sort of extensibility points and automation model Visual Studio provides without needing the Visual Studio SDK / VSIP, be sure to check out these samples:

http://www.microsoft.com/downloads/details.aspx?FamilyID=79c7e038-8768-4e1e-87ae-5bbbe3886de8&displaylang=en