Update: If you can't find the source code in the article to download, you can also get it from here.

It took a looong time and some pretty heavy paperwork, but my first ever MSDN article is up and about on the ASP.Net dev center:

Creating a plug-in framework (the link on the asp.net site)

And this is the direct link into the article on the MSDN library:

http://msdn.microsoft.com/library/en-us/dnaspp/html/pluginframework.asp

Thanks, Kent :)