WordPress.org

1. Login as an existing user, such as admin.
2. Upload the folder named ad-auth to your plugins folder, usually wp-content/plugins.
3. Activate the plugin on the Plugins screen.
4. Enable the "Admin SSL" plugin (or anything that redirects the logon page to an SSL connection) so that your passwords are not sent in cleartext.

Note: This plugin has only been tested with WordPress 2.5.1 and above, and I do not think it will work on older versions of WordPress.