This module provides a simple single sign on solution based on OpenID and native in Drupal. It is single the provider (server) module, related to: OpenID Single Sign On Relying Party

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 authentication is happening.

This module accepts sign on requests and redirects back to the relying party the user came from, authenticating the user via OpenID.

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