drush

I've recently been digging into the Aegir Hosting System, both because we're starting to use it at my current NREL gig, and because I've proposed to do a session on it at DrupalCamp LA. In short, Aegir allows you to easily administer a number of Drupal sites from a common site, itself built on Drupal. It's really slick, and a lot of the functionality is built on Drush. (Also see http://drupal.org/project/drush .)