Speedboxer

This plugin will convert valid <a href="http://twitter.com/" title="Twitter" rel="nofollow ugc">Twitter</a> usernames mentioned in comments into a link to their Twitter profile page. For example, if somebody mentions “@mattfreedman” in their comment, this plugin will check with Twitter to see if the user “mattfreedman” exists (which is does) and when it does, it will convert it into “<a href="http://twitter.com/mattfreedman" rel="nofollow ugc">@mattfreedman</a>“.
Please note: As this plugin converts usernames on the fly (whenever and everytime somebody views the comments), and checks Twitter for every username (to ensure it’s valid), I strongly recommend using a caching plugin such as <a href="https://wordpress.org/extend/plugins/wp-super-cache/" title="WP Super Cache" rel="ugc">WP Super Cache</a>. Which will not only lessen the load on Twitter’s servers, but also speed up page loads.
Although it says this plugin requires WordPress 2.6, I see no reason why it wouldn’t work with earlier versions, I just haven’t tested it with earlier version.
This plugin requires cURL to be installed on your server, and has only been tested with PHP 5.2.x.

Twitter Links

Converts Twitter usernames mentioned in comments (eg. @mattfreedman) with a link to the mentioned user's Twitter profile page.

Willy Bahuaud

“Mention comment’s authors” is a plugin that improves the WordPress comments fonctionality, adding a response system between authors. 
When adding a comment, your readers can directly mentioning the author of another comment, like facebook or twitter do,using the “@” symbol.
This mention plugin add two features :
<ul>
<li>In the comments field, when an user entered the “@” symbol, it allows, through an autocompletion system, to quote (or poke) a preceding commentator.</li>
<li>Once comments validated, the mentioned names take the appearance of buttons. When the user clicks on it, window scrolls to the preceding comment from the person named. A class is added to it, for temporarily customize it in CSS.</li>
</ul>
This WordPress plugin is based on <a href="https://github.com/tactivos/jquery-sew" rel="nofollow ugc">“jquery-sew” jQuery plugin</a>, by <a href="https://mural.ly/" rel="nofollow ugc">mural.ly</a>.
You can find more information on this post : <a href="http://wabeo.fr/blog/systeme-reponse-commentaires/" rel="nofollow ugc">wabeo : Un système de réponse dans les commentaires</a>

Mention comment's Authors by Wabeo

Helpfull to avoid nested comments and perfect for your SEO (I managed to remove a large number of useless links in my comments thanks to this plugin).
Bonus : A one-click setup ! ;)



Great tool

When adding a comment, your users can directly mentioning the author of another comment, like facebook or twitter do,using the "@" symbol.

Twitter Links vs Mention comment's Authors by Wabeo