Rules Forms Support allows users to manipulate forms when a form is built, during validation, or when a form submitted. Once events are activated for a form, users can define rules to accomplish tasks like validating form element values, setting form errors, redirecting users, or setting form titles. It was originally developed for and shipped with Rules 1 (Drupal 6), but since the release of Rules 2 (Drupal 7), the Rules Forms Support project has been moved to a separate, standalone module.
(D6) Form support
Rules forms support ships with Rules and allows you to manipulate/customize forms on your site with Rules. You have to enable events for a form that you want to use with rules (see the README.txt how to do that), then you get an event when a form is being built, during form validation and when a form is submitted.
You have many possibilities how to react on those events: you can check the value of a form element in a condition and you can use several actions: disable, hide or adjust weight of a form element; set the default value of a form element; insert prefix/suffix HTML code or set a form error.