Social Auth is an open source and free PHP library that handles this process very easily. In order to simplify sign-in/sign-up processes and reach user data instantly, authenticating + authorizing users through social networks is a highly-used.
It has support for 3 major networks: Facebook, Twitter and Google+. The library is not limited to registration + login but can also pull user data where supported.
Requirements: PHP
Website: http://docs.sonhlab.com/social-auth-for-php-app/
Demo: http://demo.siterepo.com/free/sonhlab-social-auth/
