For quite some time, I've wanted to integrate a particular webform with PayPal, since many nonprofits I help use that payment service. With older versions of PayPal, one could add some PHP code into a webform on the site to do this, but it was (a) hackish, and (b) a much less maintainable and secure way of accomplishing the goal I was trying to achieve. So I never did it.
However, after reading Additional processing in Drupal's Webform 3 module (from Drupal Coder), I found that Webform 3 has a hook that runs just after webform has saved a form's data to the database, but before the webform returns the user to a predefined redirected page.
hook_webform_submission_insert() is the perfect time to hook into Webform's process and send a user to PayPal, along with that user's data.