This module provides a simple single sign on solution based on OpenID and native in Drupal. It is the relying party counterpart for a server based on OpenID, related to: OpenID Single Sign On Provider

You can set up a central provider (which ideally is another instance of Drupal) and a lot of another (Drupal) websites (so called relying parties). This way the users can login to every single relying party website using a centralized login provider where authentification is happening.

This module overrides the general Drupal login funcionality and redirects the user to the centralized OpenID provider. The user can login there and gets send back to the website he/she was about to login.

This module is used in a more sophisticated setup (that we call Omniauth). Check the screencast on how to set it up.