OAuth
OAuth is an open standard for authorization.
OAuth allows users to log into third-party websites using their credentials from Facebook, Google, Twitter, or other major websites, without sharing their password with the third-party. It also enables the user to grant a third-party application limited permissions on their account -- so the application can read their profile data or post on their behalf.
Implementing OAuth for your site outsources the hassle of implementing a full authentication system to a trusted third-party. It is also a convenience to users, since it gives them one less password to remember.