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.

DEPRECATION NOTICE: Twitoaster and its API will be shut down this Sunday (March 20th). That means this WordPress Plugin will cease to function (like any other service relying on Twitoaster API). <a href="http://blog.twitoaster.com/twitoaster-shutting-down" title="Twitoaster shutting down" rel="nofollow ugc">Read the full Announcement here</a>
Twitter Comments system. Give to you readers the possibility to send comments via Twitter! Automatically retrieve Twitter Replies and/or Twitter Retweets to your Blog’s Posts. These Twitter mentions are handled like Posts Comments, using WP built-in comment system.
Here is how Twitter Comments work:
<ol>
<li>You Post an article on your blog</li>
<li>You update your Twitter status (or let the plugin do it for you)</li>
<li>You automatically get all related Twitter Replies and/or Twitter Retweets as comments to your Post</li>
</ol>
The plugin also brings two widgets:
<ol>
<li>Analytics charts showing how many Twitter Replies and Twitter Retweets you are receiving</li>
<li>Listings of your 20 most recent Twitter Conversations</li>
</ol>

Twitter Comments – Twitoaster

Twitter Comments system. Automatically retrieve Twitter Replies and/or Twitter Retweets to your Blog's Posts using WP built-in comment system.

Twitter Links vs Twitter Comments – Twitoaster