Sometimes I'm very thankful when a application is extensible. It allows you to customize a application or event extend a application in a easy way. Otherwise, if the code is public, I have to digg through the code and search for a place where I can add my functionality. If I'm speaking about extensible I mean it in a technical manner where normally dependency injection frameworks will be used. But what do you use in a small application where you don't have MEF, some "old" .NET 3.5 app, and a dependency injection framework like Structuremap is too big or the know how is missing?