ThemeShock

Custom 404, helps you customize your 404. 
From the dashboard options you’re able to modify the background, the image and the fonts color and style.
This page will show users the last post on your site, so that they may choose to follow one of this instead of leaving your page.
<h3>Features</h3>
<ul>
<li>Customizable background</li>
<li>Several Patterns</li>
<li>Multiple fonts</li>
<li>24 additional images.</li>
<li>5 Different languages</li>
</ul>

Custom 404 Error Page: Unilimited designs and colors

Custom 404, creates a 404 page that is set to the default one of your WordPress installation.
This page is fully customizable from the dashboard.

Jack Arturo

This plugin sends you an email notification whenever a fatal error (or other error level, configurably) is detected on your site.
Unlike traditional uptime monitoring services, which will only notify you if your entire site is down, this plugin can notify you when an error is detected on any page or process on your site.
Automatic plugin and theme updates often introduce problems that you aren’t aware of until they’re reported by your visitors. Fatal Error Notify lets you address these issues as they occur and before they cause significant problems.

Fatal Error Notify

Great plugin

Thiss plugin is useful but won´t work on certain installs, although this could easily be fixed with some enhancements.

useful plugin but could be improved

I used Fatal Error Notify on a site that kept crashing and I couldn't find out what was going wrong.  The plugin emails the PHP error message and that helped me track down the issue. 

Can be a real lifesaver

Just came here to say that this is an amazing plugin and it's extremely helpful during troubleshooting.

keep up the good work!

Extremely helpful plugin

just discovered this little nugget and I'm very happy with it!

Awesome helper

Receive email notifications when errors occur on your WordPress site.

Custom 404 Error Page: Unilimited designs and colors vs Fatal Error Notify