PeoplesGeek

Monitoring services like www.siteuptime.com or www.montastic.com require a page on your website to monitor.
This plugin provides a virtual target for your monitoring services that does not invoke other plugins (specifically avoiding the Google analytics plugins so that your statistics are not skewed by monitoring calls) 
The two targets provided for the monitoring service are:
<ul>
<li>monitor.htm – via .htaccess and does not use WordPress (but does use PHP)</li>
<li>monitorWP.htm – via WP and so tests the Database etc is working</li>
</ul>

PG Monitor

A virtual target file to be read by monitoring services instead of using a static HTML page.

nabtron

Show average server load and uptime of your linux server on top in admin panel. It contains three values, first one is the load average for last 15 minutes, second one is for 5 minutes, third one is for last 1 minute.
Now you can easily monitor your server load in real time whenever you want without installing any script or moving out of your wordpress admin panel.
Options include turn off, on or legacy mode.
It shows:
<ul>
<li>3 averages for server load</li>
<li>server uptime in days and hours</li>
<li>Memory usage in %age</li>
</ul>
If useful – Future versions are planned to have more options, so please leave a feedback
Found a bug or have a feature request ? <a href="https://nabtron.com/wp-server-plugin/" rel="nofollow ugc">Report here</a>

WP Server

Show average server load and uptime of your linux server on top in admin panel

PG Monitor vs WP Server