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.

Nazmul Hasan Robin

This plugin allows you to show youtube subscribe button anywhere in your website using widget.
Major features in Social Subscribe – Follow Buttons include:
<ul>
<li>Shows youtube subscribe button</li>
<li>Shows youtube subscribers count </li>
<li>Settings from backend to change design </li>
<li>Available in widgets </li>
<li>Other Social Networking Site’s buttons are coming soon!</li>
</ul>

Social Subscribe – Follow Buttons

This plugin provides a widget to show a youtube subscribe button. Also number of youtube subscribers can be displayed.

Twitter follow button in coments vs Social Subscribe – Follow Buttons