WordPress.org

1. Login as an existing user, such as admin.
2. Upload http-authentication.php to your plugins folder, usually wp-content/plugins.
3. Activate the plugin on the Plugins screen.
4. Add one or more users to WordPress, specifying the external username for the Nickname field. Also be sure to set the role for each user.
5. Logout.
6. Protect wp-login.php and wp-admin using your external authentication (using, for example, .htaccess files).
7. Try logging in as one of the users added in step 4.

Note: This version works with WordPress 2.5.1 and above. Use version 1.8 if you are using WordPress 2.0.

This plugin also supports WordPress MU as of version 2.4. You can install it globally or on a per-site basis. Refer to the WordPress MU documentation for more information on installing plugins.