The CLR team is introducing a cool new framework in Orcas that makes it easier than ever before for managed applications to host add-ins.  In the February 2007 issue of MSDN Magazine, Jack Gudenkauf and Jesse Kaplan from the CLR team provide a great introduction to the System.AddIn namespace; they have a follow-up article in the March 2007 issue, too, so if you're a subscriber, that should be arriving in your mailbox soon.  If you're interested in allowing other developers to write add-ins for your apps, definitely check it out. Additionally, for more information, the CLR Add-In Team also just started a blog.

-Stephen