Subscribing the new user to a mailing list as a part of registration
The email address is already a standard user property. Using fields you can ask for permission about the mailing list.

You can use rules module (http://drupal.org/project/rules) to get notified of new registrations and find or implement the action for a mailing list registration.

Another option would be the simple news module: http://drupal.org/project/simplenews