I have hacked my first WordPress plugin called ''New Comment Notify":
Comment authors, which have added a comment to a post, will be notified about new comments to this post by using their given
email address. Based upon the 'Update Notify' plugin from Keith McDuffee (http://www.gudlyf.com/index.php?p=357).
The plugin is available from http://DigitalFotoKunst.de/downloads/wp-new-comment-notify-0.1.zip