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 will allow visitor to submit they twitter, while thay are commenting your posts. They account will by showed as twitter follow button.
Registred users can add their twitter account in profile administration.
There is also a setting page where you can edit color, size, showing count and language.
Some themes dont’t use function <code><?php comment_form(); ?></code> to show comment form. In this case you have to edit your template. Find comments.php and add
<pre><code><label for="twitter">Twitter</label><input name="twitter" size="30" type="text" value=''/>
</code></pre>
to form.

Twitter follow button in coments

Allow your visitors to add their twitter.

Twitter Links vs Twitter follow button in coments