Magento, IMHO, represents a PHP system that is built on well thought-out coding principles - reuseable design patterns being one of them. In terms of an example of a PHP system, I think it can be considered pretty cutting edge and therefore worth considering from an architectural point of view.
As I understand it, there are many design patterns that are available to the OOP developer. Seeing such patterns being put to use in an open-source system such as Magento allows a developer to view examples of such patterns in real use and in situ, rather than in examples that can sometimes be rather achedemic, and even a little misleading.
As such, I am wondering what patterns, other than the ones I have listed below, Magento programmers have used when developing for Magento.
As a note, I understand that some of these patterns are in place as a consequence of being built on the Zend Framework, MVC / Front Controller being a couple of them,
The obvious ones are:
Factory:
$product = Mage::getModel('catalog/product');
Singleton:
$category = Mage::getSingleton('catalog/session');
Registry:
$currentCategory = Mage::registry('current_category');
