Phire CMS : Open Source Content Management System
Extending : Plugins
Improve how it works.
Enter plugins. Here, the focus is on how it functions. So developers don't have to worry so much about HTML and presentation (that's reserved for themes), and can focus on the PHP code necessary to get the plugin stable and functional.
Of course, plugins can be developed in any number of ways. The quickest and easiest way is to build on top of the API of the system that's already in place. Or, if you're comfort level is with another method of developing code, then you can go down that path as well.
Obviously, since Phire was just launched, there are but only a few plugins available, but, hopefully this will change once the Phire Developer Center launches.