An experimental module to enable true private downloads for Drupal 6.
If you simply require .htaccess protection read the handbook page here:
Restrict specific folders from public download (via .htaccess)
Drupal Handbook - http://drupal.org/node/540754
Or try one of the following private downloads related projects
From Drupal 7, both private and public downloads will be supported by core.
About the 2.x branch
It reworks how the url is done to be more flexiable with contrib modules. It does optionally use htaccess, but storing the files out of the webroot is still preferable.