Allow your visitors to add their twitter.
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
<?php comment_form(); ?> to show comment form. In this case you have to edit your template. Find comments.php and add
<p><label for="twitter">Twitter</label><input name="twitter" size="30" type="text" value=''/></p>