You can go a long way to make a large application customisable. You can include a comprehensive options and preferences system or even use configuration files to allow access to advanced settings, but there's nothing like being able to write code within an application to fully control it or simply hook in to it.
I recently wrote an IRC client in .NET. People have grown accustomed to having scripting support in such things so as part of the exercise I decided to include exactly that.
How Scripting is Implemented
Scripts are implemented in a similar way to plugins. Essentially, they ARE plugins, but they're compiled on the fly from inside the application rather than precompiled. This makes it much easier for the user since they don't have to worry about unloading existing plugins, possibly closing your application then recompiling their dll.
Plugins work by conforming to an interface defined in a shared assembly that the host application knows about. Often when the plugin is initialised a reference is also passed to it of another interface that the host application has implemented, to allow the plugin to control the behaviour and other aspects of that application.
Luckily for us, in .NET the compilers for the main languages are built right in to the framework. These are worth an article on their own really, but for the purposes of this article we will but scratch the surface. The key to us using them for this purpose is that they can accept language source as a string and spit out an assembly in memory. This assembly object acts just the same as an assembly object loaded from a compiled dll.
By the end of this article I hope to have taken you through the process of doing this, and have developed a helper class in both VB and C# that illustrates the processes involved.