How do you enable customers to log in to your site using their Google account?
See here: Google Login PHP Class.
Also be sure to refer to the Google Federated Login site for more info.
You may want to look at this too: https://rpxnow.com/ - it will only need integrating at the HTML/javascript level.
It's what http://uservoice.com/login appears to use.
You should look at the OpenID Enablded PHP library (http://www.openidenabled.com/php-openid/).
This should play pretty nicely with any LAMP installation without needing to use Zend.