denishua

When a reply is made to a comment the user has left on the blog, an e-mail shall be sent to the user to notify him of the reply. This will allow the users to follow up the comment and expand the conversation if desired. 
Detail information: http://theme10.com/comment-reply-notification/
中文介绍： http://fairyfish.net/2008/11/03/comment-reply-notification/

Comment Reply Notification

When a reply is made to a comment the user has left on the blog, an e-mail shall be sent to the user to notify him of the reply.

Yakup Hoca

The visitors send their questions in comments, then they leaves our web sites. When we reply their comment to answer their questions, this plugin sends an e-mail about this. So that, the visitor will learn this situation and return to our site to read our answer.
For support: <a href="http://www.yakuphoca.com/comment-reply-by-admins-notifier-plugin/" title="Support by Yakup Hoca" rel="nofollow ugc">Plugin URI</a>

Comment Reply by Admins Notifier

This plugin sends an e-mail notify when an admin or an editor of site reply a comment.

Comment Reply Notification vs Comment Reply by Admins Notifier