Single Sign-On as defined on Wikipedia is "a property of access control of multiple related, but independent software systems". In this context Drupal is most often the service provider, the system on which the user wishes to access resources that may be restricted based on the users identity (roles, permissions, per user access control etc). Note that SSO is different than OpenID, Facebook Connect, etc that simply provide the authentication mechanism with which a login attempt on a site that uses these technologies will be processed. With SSO, a user, once logged in on one of the cooperating sites, is immediately logged in on the othes site as well.
In SSO systems a third party service offers the identification and authorization mechanism and is responsible of informing Drupal that a given users identity can be trusted.
The following modules provide Single Sign-On support for Drupal (list is a work in progress, feel free to edit it):
- Janrain - integration with the commercial Janrain service
- Google Apps Authentication
- SimpleSAMLphp Auth - integration with the open source library SimpleSAMLphp
- HybridAuth Social Login - integration with the HybridAuth library
- Bakery - SSO among Drupal installations on the same parent domain
- Account SYnc - provides SSO functionality and account info synchronization amond Drupal installations.