IndieAuth!

OAuth is an open standard for authorization. It allows users to share their private resources (e.g. photos, videos, contact lists) stored on one site with another site without having to hand out their credentials, typically supplying username and password tokens instead. Each token grants access to a specific site (e.g., a video editing site) for specific resources (e.g., just videos from a specific album) and for a defined duration (e.g., the next 2 hours). This allows a user to grant a third party site access to their information stored with another service provider, without sharing their access permissions or the full extent of their data. »

Portier (pronounced "Por-tee-ay") is a self-hostable login service that you can use instead of passwords. Portier sits between your website and third-party services like Google Sign-In to provide your users the fastest and easiest login experience, without ever needing a new password.

Janrain Engage (formerly RPX) enables you to quickly register and login users on your site via their existing Facebook, Google, Yahoo, Twitter, or OpenID accounts. Import rich user profile data, and publish user activities from your site to multiple social networks. Increase registration rates, generate qualified referral traffic from the social networks, and grow brand awareness.

Place a button on your site or application which allows Twitter users to enjoy the benefits of a registered user account in as little as one click. Works on websites, iOS, mobile and desktop applications.

Facebook Connect is a set of APIs from Facebook that enable Facebook members to log onto third-party websites, applications, mobile devices and gaming systems with their Facebook identity. While logged in, users can connect with friends via these media and post information and updates to their Facebook profile. Developers can use these services to help their users connect and share with their Facebook friends on and off of Facebook and increase engagement for their website or application.