Form Tweak [formtweak] makes lots of useful little modifications to Drupal form pages easy, using a non-database configuration method. Currently features renaming submit buttons and adding a save-and-edit button.
See Agaric wants a "Save and edit button" button for all content forms for background. (An earlier iteration featuring a disabling of the delete option for regular users is obsolete for Drupal 6.)
See the formfilter module for a different approach and for altering most everything except submit buttons.