Bakery Single Sign-On System
Bakery enables single sign-on across multiple Drupal sites hosted on the same second-level domain. It allows users to log into one site and be automatically logged into other connected sites. This functionality can also extend to non-Drupal websites that support compatible authentication methods.
Bakery provides a "single sign on" feature for Drupal based sites that are on the same second-level domain (i.e. example.com, subsite.example.com, subsite2.example.com). It could also provide support for any other website that implements the same web cookie, xmlrpc, and POST methods.
Read the Bakery documentation for more information.
Note, Bakery 1.x is incompatible with Bakery 2.x. Run the same releases (including same daily development tarball) on all domains you wish to provide SSO for.
Vagrant and Chef setup for testing Bakery
Development credits
The original design and code are by David Strauss. The module is currently maintained and developed by some members of the Drupal.org infrastructure team and some people who use it on their sites or their customer's sites. Currently seeking new maintainers.