Take for example the following use case – the standard DB resource plugin instantiates a database connection based on supplied parameters and registers it with Zend_Db_Table_Abstract::setDefaultAdapter(). What if you want to register the connection in the Zend_Registry as well? You could create a bootstrap method (_initRegistry() for example) and pull the adapter out of Zend_Db_Table_Abstract or you could simply extend the resource plugin.

// library/My/Application/Resource/Db.php
 
class My_Application_Resource_Db extends Zend_Application_Resource_Db
{
    public function init()
    {
        if (null !== ($db = $this->getDbAdapter())) {
            Zend_Registry::set('dbAdapter', $db);
        }
        return parent::init();
    }
}

This class simply adds the created database adapter to the registry before continuing on with the default behaviour.

To let the application know about your custom plugin, you simply add the plugin path to your application.ini file, eg

includePaths.library = APPLICATION_PATH "/../library"
pluginPaths.My_Application_Resource = "My/Application/Resource"

More reading is available here – http://framework.zend.com/manual/en/zend.application.theory-of-operation.html

Further examples here – http://framework.zend.com/manual/en/zend.application.examples.html