I need some resources that talk about how to design your software to be extensible, i.e. so that other people can write add-ons/plug-ins that adds functionality to it.

What do you recommend? Any books out there that discuss the subject?  
I would prefer something that&#39;s short and to the point; a bit of theory and a bunch of concrete examples.

I&#39;m not targeting a specific language, I want to be able to understand the core idea so that I can implement it in any language.  

And for the same reason, I prefer not to do it using a framework that someone else built (unless the framework is not very high-level, i.e. doesn&#39;t hide _too_ much), at the moment I only want to educate myself on the subject and experiment with various ways to implement it. Plus, a framework usually assumes user&#39;s knowledge about the subject.

**UPDATE**

I&#39;m not asking about OOP or allowing my classes to be inherited. I&#39;m talking about designing an application that will be deployed on a system, such that it can be extended by third-party add-ons AFTER its been deployed.

For example, Notepad++ has a plug-in architecture where you can place a .dll file in the plugins folder, and it adds functionality to the application that wasn&#39;t there, such as color-picking, or snippet insertion, or many other things (a wide range of functionality).

All Extensibility Solutions on Gang of Coders

Total of 1 Extensibility Solutions

How to design extensible software (plugin architecture)?