NerdKey

Powered by NerdTools.co.uk Uptime Tracker (http://uptime.nerdtools.co.uk/) this plugin will add a widget to your sidebar that will show the percentage of availability or uptime for any website over the last 30 days.
In terms of the tracking system, checks are carried out every 5 minutes from different locations and then averages are worked out to form the overall reports from which the graph is generated and shown via an iframe.
Other reports including hourly, weekly and daily can be seen by visiting the Uptime Tracker website directly.
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Uptime Widget

Adds a simple uptime gauge to your WordPress sidebar, showing the percentage of availability for any website over the past 30 days.

Daisuke Takahashi

Do you want to monitor your server without using SSH?
Don’t you know how to use difficult commands? (looks like a spell!)
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OK! Leave all to this plugin!
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“Server Status” adds widget like ‘uptime’ command in Dashboard and Network Admin Dashboard.
<h3>Notice</h3>
<ul>
<li>Currently, only RHEL/CentOS is tested. And OS X is tesing now! (The number of tested OS will increase shortly.)</li>
<li>PECL Zend OPcache users, please add server-status.php to black list! Otherwise, there must be segmentation fault. 
This troublesome process isn’t required on PHP5.5!</li>
<li>PHP 5.2 users, your version of PHP is no longer supported.(<a href="http://www.php.net/eol.php" rel="nofollow ugc">Detail</a>) If you’re using such older version, this plugin nags at it!</li>
</ul>

Server Status

thank you a lot for this work, keep it up!

a little too much info, but great for monitoring while dev/stage

Show server information widget in Dashboard and Network Admin Dashboard.(Currently, only RHEL is tested)

Uptime Widget vs Server Status