HybridAuth is a library for web apps which implements social sign-ins (meaning, you authenticate with an existing social network account (like Twitter or Facebook)) rather than applying for an account with an e-mail address. Lets the user import established profiles and contact lists from the socnet account they logged in with. Also has a library of plugins for existing frameworks and applications, from Drupal to Concrete to Wordpress. Written in PHP so that it'll run on just about every webhost out there out of the box.