Zend Framework is a popular PHP Framework on Ohloh.net and is used by Tiki since Tiki3. We will upgrade to Zend Framework 2 when it's released.
Tiki is the or among the most popular FOSS projects that uses Zend Framework.
Development:
http://dev.tiki.org/Zend+Framework
Related links:
http://framework.zend.com/
http://en.wikipedia.org/wiki/Zend_Framework
https://www.ohloh.net/p/zend_framework