wesg

Comment Connection is a plugin that turns an element of comment text to a link for a previous comment. Whenever an author uses @commenter, the text after @ is automatically linked to the previous comment it refers to.
The plugin works by searching each comment for an occurance of @. Where it finds one, the plugin searches the comment database for a comment on the same post written by the person being referenced. By doing it this way, the plugin only adds one database query for each referencing comment.
If the type after the @ symbol does not correspond to a comment, no changes are made.
Currently tested successfully with the most current version of WP, 2.7. If you have trouble with earlier versions, or even have it work properly on earlier versions, please let me know.
For a complete list of the changes from each version, please visit <a href="http://www.wesg.ca/2008/04/wordpress-plugin-comment-connection/#changelog" rel="nofollow ugc">the plugin homepage</a>.
Be sure to check out my other plugins at <a href="https://wordpress.org/extend/plugins/profile/wesg" rel="ugc">my WordPress profile</a>.
<h4>USAGE</h4>
Inside a comment, write @commenter where commenter is the name of the author you are replying to. The plugin will successfully find a link when the commenter text is followed by a new line, colon (:), comma (,) or a dash (-).
<h4>LIMITATIONS</h4>
<ol>
<li>Commenter must be an exact match of the original author’s name.</li>
<li>Many database queries can occur for a post with many referencing comments.</li>
<li>Only links to the last comment made by commenter (ie. if the commenter writes many comments before the @, the most recent one is linked to).</li>
</ol>

Comment Connection

Comment Connection is a Wordpress plugin that automatically links comments as authors reply to each other.

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.

Comment Connection vs Comment Reply Notification