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.

This plugin allows you to add Twitter-like @reply links to comments. 
When clicked, those links insert the author name and a link to the comment you are replying to in the textarea.
Most of the code is taken from the Custom Smilies plugin by Quang Anh Do which is released under GNU GPL : 
https://wordpress.org/extend/plugins/custom-smilies/
Thanks to Guillaume Ringuenet for the arrow graphics.
I didn’t create anything new, and I do not claim so in any way. 
This plugin is just a feature I wanted and judging by the number of downloads I guess I wasn’t the only one. 😉

@ Reply

This plugin allows you to add Twitter-like @reply links to comments.

Comment Connection vs @ Reply