At work, we make heavy use of our central LDAP directory for Drupal authentication. For our Drupal 6 work we use the LDAP Integration module but there's no version of that module for Drupal 7. So far, we've been using the LDAP module which is on track to becoming the go to module for LDAP in Drupal 7. Right now, the module's listed as unstable, but they've got a strong start and the Server and Authentication components work quite well.
