nickgrossman

Enable commenting on your posts via Twitter.
Posts will get a “Discuss on Twitter” button, which will prompt visitors to reply to the original tweet about the post, as well as a “View Discussions” button which will link to existing tweets about the post.
Each new post will be auto-tweeted (requires the <a href="https://wordpress.org/plugins/wp-to-twitter/" rel="ugc">WP to Twitter</a> plugin and a <a href="https://developer.twitter.com/" rel="nofollow ugc">Twitter Developer account</a>).

Discuss on Twitter

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.

Discuss on Twitter vs Twitter Links