The use of plug-ins or add-ins are a common way of extending any application, most people do not even know when they are using an extension.
I am currently using an add-in framework in a project that I am working on and It has proven its self invaluable. The framework I speak of here is baked right into the .net Framework from 3.5, The Managed Add-in Framework being fairly new and has a few glaring issues that need to be addressed like the naming conventions and directory structure that must be adhered to avoid making the entire framework come crashing down.
In order to use the framework easily you will need to download the Pipeline Builder CTP March 2008 which is the latest release at the time of writing, please check the clr addins blog or clr addins homed on codeplex.
While there are annoyances with the state of its development, some of the features you get from using it out of the box are nice. The ability to launch add-ins either cross process or cross domain is great.
The add-in framework uses Adapters on either site of the add-in contract or isolation boundary as proxies. This is some of the magic that also enables efficient version control of the add-ins without the changing or breaking the add-ins.
For Additional information I would strongly recommend checking out these two videos
also further reading relating to the Managed Addin Framework or MAF look here.
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