ZendFramework integration with other frameworks

I use Doctrine 2 in my Zend Framework projects. It's easy to use and fits in seamlessly. The basic steps to integrate it are:

• Write a resource plugin that will initialize a Doctrine 2 EntityManager using configurations options in application.ini. An example of one can be found in Benjamin Eberlei's DoctrineExtensions.
• Rather than rely on a global registry (Zend_Registry) to store the EntityManager reference, I rely on a simple depedency injection solution as described here to provde the instance to my controllers.
• I rely on Doctrine's Event system to create re-usable 'behavior' classes to do things like update timestamps or validate data.
• I keep all my Entities and business objects in my application's 'models' folder (under it's own namespace), and everything else in in my library folder. I heavily use PHP 5.3 namespaces to keep things organized.
• I use Doctrine's new-ish Migrations library to handle updates to the database via the command line.
• I also highly recommend creating a detailed build process in Phing in order to handle library dependencies, automated unit testing and running migrations that can be integrated into source control or a CI Server. Also good for compiling Javascript or CSS if you're using one of those frameworks.

These practices aren't just good for Doctrine, they'll be handy anytime you're trying to built a site's architecture. I think that Zend's biggest assets in extensibility are Resource plugins and ActionHelpers / Controller Plugins.