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.

mcalamelli

The BTCNew WordPress plugin finds conversations related to your content and allows you to display the associated comments inline with your own. Existing conversations about your content taking place across the web can now be displayed right on your blog for your visitors to read and respond to. It works on top of your existing WordPress comments, both threaded and non-threaded.
Currently, we support conversations from:
<ul>
<li>Twitter – tweets that link your posts</li>
<li>FriendFeed – on the entries for your posts</li>
<li>Digg – on the submissions for your posts</li>
<li>Reddit – on the submissions for your posts</li>
<li>Hacker News – on the submissions for your posts</li>
<li>Other blogs – whenever someone links one of your posts</li>
</ul>
The plugin is highly customizable. You can:
<ul>
<li>Specify which sources you want to include comments from</li>
<li>Add a summary to the top of your comments section, which shows the number of comments from each source</li>
<li>Specify whether you would like BackType Connect comments to be displayed at the end of the comments section, or mixed with the rest of your comments</li>
</ul>
This plugin was created using <a href="http://www.backtype.com/developers/connect" rel="nofollow ugc">BackType Connect</a>. 
This plugin is a fork of the old Backtype Connect WordPress plugin, developed by Backtype.
<h3>Support</h3>
If you’re having problems using our plugin, let me know. I’m happy to help! You can contact me through <a href="http://www.mcalamelli.net" rel="nofollow ugc">my website</a>.

BTCNew

The BTCNew Wordpress plugin lets you show related conversations (from Twitter, Digg, FriendFeed & more) inline with your own comments.

Twitter Links vs BTCNew