Drupal authentication for a non-Drupal script I have a non-Drupal PHP script I would like to put on a D7 Drupal site, but I would like it to be available only to users who have logged in via Drupal. What are the best/easiest ways to do this? It's my script, so I can change it any way I want, embed it in a module, a PHP-capable field, etc.

To be specific, it's a signup sheet system that uses form submissions, using files (not the db) to store the data.
You can wire up your script via a hook_menu, and instead of returning the output to render via Drupal, you can print it directly and exit at the bottom of the function.  The submission handler would do the same thing, but drupal_goto() at the end.

That said, this is a moderately bad idea for the long run.

A better solution would be to use Webforms.  Webforms are really easy to make, get stored in the database, and can be exported to Excel.  I wouldn't be surprised if there is webform hook available so you could do your filesystem thing, too.

Another option is to use the Forms API to make your form w/ your custom validation and submission handlers.