System.AddIn in Framework V3.5 is about building hosts that load plug-in Add-Ins with functionality around discovery, isolation and lifetime management. It also tries to deal with the thorny problem of versioning one or both ends of the host<->addin relationship.

I didn't know too much about the thorny details of this so I thought I'd forward the links I used to learn a little more about it (read in the following order);

MSDN Magazine Article 1

MSDN Magazine Article 2

Approaches to Discovery

How Discovery Works (I wish MS folks would write more articles like this. That is, my view is that I just want to know "What code did you just build into your framework piece and how does it do what it does?". If you tell me that then I can figure the rest).

Architecture This I found interesting only for the statement "The host and add-in only ever depend on one component (the view) which itself has no dependencies elsewhere on the system. This means that the host and add-in are completely unaware of each other and the contracts that are used to cross the boundary". To me, that is starting to explain why we have so many assemblies involved in building an AddIn - it's to avoid dependency (sure, this is probably obvious to you already).

Process Isolation Introduced

Adapters

How Activation Works

How to Build an Add-In

At the time of writing, I'm still reading :-)