Sometime back I had posted about writing applications that can load plugins using late binding. Users can drop assemblies in a specific folders which is scanned by an host application at startup. Using reflection the host can locate classes that implement a specific interface and instantiate those classes as plugins. The drawback is that the host processes plugin assemblies and you need to store/maintain the corresponding sources elsewhere.

nice article by Abhinaba

http://blogs.msdn.com/abhinaba/archive/2006/02/09/528416.aspx