Awesome

Single Sign On (SSO) Client for PHP

The official PHP SSO Client for the Barebones SSO Server.

Barebones SSO Server is an awesome, scalable, secure, flexible login system.

Average memory footprint. About 1MB RAM per connection.
Classes and functions are carefully named to avoid naming conflicts with third-party software.
When authentication is required prior to executing some task (e.g. posting a comment), the SSO client encrypts and sends the current request data ($_GET, $_POST, etc.) to the SSO server for later retrieval and will resume exactly where it left off in most cases (e.g. the comment is posted).
Encrypts communications over the network (even HTTP).
Communicates with the server on a schedule set by the client. Allows for significantly reduced network overhead without affecting system integrity.
And more. See the full feature list.
Also has a liberal open source license. MIT or LGPL, your choice.
Designed for relatively painless integration into your project.
Sits on GitHub for all of that pull request and issue tracker goodness to easily submit changes and ideas respectively.