Lifting the Skirt on Zend Framework 1.5: Zend_Auth and the OpenId Adapter.
Probably my one favorite aspects of Zend Framework is that you can pull pieces out and use them by themselves without having to build a complete MVC installation. Not that building out a complete installation is difficult but there are times when you just need a small piece, not the entire puzzle. Zend_Auth is one of those pieces that can be pulled out and used by itself. If you need authentication for an existing application Zend_Auth may just be the piece you are looking for.
Via: Zend Developer Zone