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.

Michal Ozogan

This plugin fetch all tweets from Twitter, which has been written about some of your posts. For new article are changes cheching every hour. For oldone it is question one or two days.
You can set how many tweets to show and in swich format. It can be regular HTML or embed tweets generated by Twitter javascript.
List of tweets is automaticky show above comment form, but your can also do manually by put funciont <code><?php tmip_show_tweets() ?></code> into your WordPress theme.
Just remember, Twitter allow to search tweets only few days old, so i will show probably only in new articles. Be patient.

Twitter mentions in posts

Show tweets about your posts right under them.

Twitter Links vs Twitter mentions in posts