If you have forms on your page that contain default text in them, this is a great time-saving script that makes life easier for your visitors that have to fill them out. The script automatically clears the default text inside the containing form element when the user sets focus on it, so the user can just aim, focus, and starting typing. Cool!
A good addition to your forms if they contain a multitude of check/radio buttons is a "check/uncheck all" feature. This script does just that for any group of checkboxes and radio buttons.