Drupal 5.x has a module 'webserver_auth' that allows native webserver authentication methods to be used to authenticate users for Drupal. Unfortunately this module has not yet been ported to Drupal version 6.x; this patch attempts to rectify this problem.
This patch fixes up the webserver_auth module to work with Drupal version 6.x. Fortunately it's quite a straightforward change.
Here is the most recent version of the patch:
All versions of the patch are listed here. Some may work better than others.