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.

benjiballin

The Twiogle Twitter commenter plugin pulls in comments from Twitter to your own blog posts. 
This plugin will look at every tag in your post and try to pull tweets from twitter based off of your tags and post that tweet as a comment on your post. 
Be sure to tag your posts with good tags, do not tag your posts with an unfamiliar word or it may not pull any comments
<ul>
<li>gets your archived posts and pages re-indexed because content will be ever changing</li>
<li>manages the organic page growth of your autoblogs</li>
<li>injects targeted (based on your tags) content and keywords into your posts</li>
<li>encourages real readers to add their own comments, no one wants to be the first one to comment</li>
<li>removes the zero comment footprint common to most blogs</li>
</ul>

Twiogle Twitter Commenter

The Twiogle Twitter commenter plugin pulls in comments from Twitter to your own blog posts.

Twitter Links vs Twiogle Twitter Commenter