Zend Framework
Zend Framework is a PHP framework surprisingly managed by Zend.
Unlike other frameworks like Symfony and Code Ignitor its built using a ‘use-at-will’ architecture which means that you can use either part of all of the framework depending on your situation.
I got involved at an early stage of the framework and spent some time contributing code, unit tests, documentation and bug fixes.
You can read more about the Zend Framework at the Zend Framework Site