Jordi Cabot

In the Settings->Discussion page, authors of a post can use the checkbox Email me whenever – Anyone posts a comment.
But this does not send an email as well to the site admins. In blogs where you have a number of guest authors, you may want to be informed about all the new comments so you can respond (if the author is missing) or just participate in the discussion.
As a site admin myself, I was missing many comments. This means plenty of missing opportunities to engage with your audience.
To solve this situation, the plugin adds a new checkbox in the Discussion page. If checked, admins will get an alert email for new comments.

Comment Admin Notifier

Very simple plugin that doesn't try to do many things. But the one it does is very useful: as a website owner you want to make sure you're on top of all the comments in the site even comments to posts you didn't write. 

It does one thing and it does it right

With this plugin, admin users get an email alert every time a new comment is posted on ANY post in the site.

Rafael Mardojai CM

Use responsives alert boxes with shortcodes. 
Is easy, only use the shortcode.
Example: For a info message use: 
 [alert type=”success”]Your text here[/alert] 
Also you can change the icon size: 
 [alert type=”success” icon-size=”big”]Your text here[/alert]
Shortcode attributes:
<ul>
<li>
type (defines the style of the alert box):
<ul>
<li>info (display a info style box)</li>
<li>success (display a success style box)</li>
<li>warning (display a warning style box)</li>
<li>danger (display a danger style box)</li>
</ul>
</li>
<li>
icon_size (defines the icon size):
<ul>
<li>normal (icon normal size)</li>
<li>small (icon small size)</li>
<li>big (icon big size)</li>
<li>hide-icon (no display the icon)</li>
</ul>
[alert type=”type” icon-size=”icon-size”]Your text here[/alert]
</li>
</ul>
<ul>
<li>Demo & Examples: http://www.rafael.mardojai.com/simple-alert-boxes-plugin/</li>
<li>Github repository: https://github.com/mardojai/Simple-Alert-Boxes-Wordpress-Plugin</li>
</ul>

Simple Alert Boxes

Nice

Use responsives alert boxes with shortcodes.

Comment Admin Notifier vs Simple Alert Boxes