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.

bingu

[ATTENTION] WordPress version MUST up to 2.7
Enhance the threaded comments system of WordPress 2.7. 
When someone reply to your comment, send a email to you.

replyMail

Enhance the threaded comments system of WordPress 2.7. When someone reply to your comment, send a email to you.

Comment Reply Notification vs replyMail