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.

metaphorcreations

Custom pack of widgets including shortcode generators.
<ul>
<li>Twitter</li>
<li>Social</li>
<li>Recent Comments</li>
<li>Recent Posts</li>
<li>Tabbed Posts</li>
<li>Contact</li>
<li>Post Navigation</li>
<li>Collapse</li>
</ul>
<h3>Advanced Info</h3>
Each widget includes a shortcode generator. Copy and paste the shortcodes into your posts to display the widget contents anywhere on your site.
Each widget also displays the widget ID of the widget. This can be helpful if you wish to use any of the filters built into the widgets.

Metaphor Widgets

Custom pack of widgets including shortcode generators.

Twitter Links vs Metaphor Widgets