Since there have been numerous tutorials written regarding Zend_Auth, I won't attempt to make a full tutorial on it. If you are curious, google around, you'll find them. Today though, I'd like to show you a cool new piece that has been added to Auth in Zend Framework 1.5, the new Zend_Auth_Adapter_OpenId.
Cal explains what the OpenID system is all about before getting into the code that uses it. He suggests MyOpenID as a provider to register at and includes the code to make a request to them to authenticate a username/password combo. The actual authentication check is only about three or four lines of code.