Drupal 7 Page Delivery Callbacks
Or, how you can render a Drupal page with an entirely different template.
I recently had a requirement where I needed to get Drupal to render a single page of HTML that was entirely separate from the normal page layout of a site. This was actually part of an API callback, but this got me involved in looking at how delivery callbacks work in Drupal 7. It isn't necessary to create a new theme just for the job of rendering a single page with some custom HTML, especially as Drupal has mechanisms to provide this built in.