People have been talking about John Van Dyke's Actions module ever since I started up with Drupal, back in the 4.6 days. It is now baked into Drupal 6, and all this time I have been kind of wondering what the fuss was about. Now, with my discovery of "Views Bulk Operations":http://drupal.org/project/views_bulk_operations I finally have a reason to get excited about actions.
This fantastic module allows you to transform any view into a set of checkboxes which you can then perform actions on. Combine a couple exposed filters and some custom actions, and the possibilities are endless. The use case that got me going was moving a subset of taxonomy tags from an obsolete vocabulary to new one. In the past I would have written a custom script, but View Bulk Op did it out of the box. Later I decided I really wanted Node Reference instead of taxonomy tags, which finally gave me an excuse to write my first action. Node Reference Action was easy enough to throw together, and now I am moving node meta data back and forth at my whim :)
I haven't released the action yet as it still has some rough edges. If you are clamoring for such a thing, be sure to leave a comment.