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.