Passport is authentication middleware for Node.js. Extremely flexible and modular, Passport can be unobtrusively dropped in to any Express-based web application. A comprehensive set of strategies support authentication using a username and password, Facebook, Twitter, and more.

Sentry is an open source PHP 5.3+ fully-featured authentication & authorization system. It also provides additional features such as user groups and security features. It's framework agnostic (composer), so it can be installed on Laravel, CodeIgniter, etc. They recently released their official docs and I really like how the interface works. Related discussion: http://www.reddit.com/r/PHP/comments/1aitay/simple_powerful_and_easy_to_use_authorization_and/

"Velruse is a set of authentication routines that provide a unified way to have a website user authenticate to a variety of different identity providers and/or a variety of different authentication schemes.

It is similar in some ways to RPXnow with the exception of being open-source, locally installable, and easily pluggable for custom identity providers and authentication schemes.

You can run Velruse as a stand-alone service for use with your websites regardless of the language they're written in. While Velruse itself is written in Python, since it can interact with your website purely via HTTP POST's."

"This library is essentially a wrapper for the Facebook PHP API, but also includes everything you need to get Facebook Connect working with in your CodeIgniter libraries."