Notify module

Notify module is a powerful way to subscribe users to periodic emails which include all new or revised content and/or comments much like the daily news letters sent by some websites. Even if this feature is not configured for normal site users, it can be a useful feature for an administrator of a site to monitor new content submissions and comment posts.

http://mydrupal.com/auto_subscribe_users_to_new_content_new_comments_to_...