#module #underrated

I was wondering if any of you had suggestions for underrated modules - the kind of modules that don't get a lot of publicity (like views), but from your experience have been excellent to use? I'm always looking for something that will help, whether it be for content management, design, whatever -

I know for me, Node Clone (http://drupal.org/project/node_clone) has been invaluable for duplicating content for our calendar.