Most of these provide some level of integration with Google's API for authorization. Some offer synchronizing Drupal and Google Apps credentials also.
DrupalGapps http://drupal.org/project/drupalgapps (6.x)
Google Authentication for Web Applications http://drupal.org/project/google_auth (6.x)
Google Apps Authentication http://drupal.org/project/googleauth (5.x)
Google Auth http://drupal.org/project/gauth (No release)
Google App Authentication http://drupal.org/project/googleapp_authentication (no code)
Comparison to come...