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.

miniOrange

You will get email notification alert as soon as your website is down and back live using this FREE, Simple & very easy to setup plugin. We use our Uptime monitoring service to check your domain regularly and alert you as soon as we find your website is down. 
This plugin will also help you monitor the website’s uptime in a given period of time using the premium plan. It would make website maintenance easier and you wouldn’t have to manually check your website’s status.
<h4>FREE Plugin Feature</h4>
<ul>
<li>Site-Uptime Monitoring: It helps you know when your website or server is down so that you can take necessary steps before your customers face issues. You can easily monitor your website’s uptime and downtime using this plugin.</li>
<li>Email Notification: We send an email alert when we detect that your website is down so that you can take necessary steps. We will also alert you when your site is back up. </li>
<li>Instant Detection: Our service checks your domain regularly so that we can alert you as soon as your website is down.</li>
</ul>
miniOrange uses login.xecurify.com if the user chooses to register and upgrade to premium. If the user does not want to register then he can continue using the free plugin. 
(Link to the privacy policy – https://www.miniorange.com/privacy-policy.pdf)
Customized solutions and Active support are available. Email us at info@xecurify.com or call us at +1 9786589387.

Site Uptime Monitoring

Simple & Easy UI. This plugin will alert you via email notifications when your site is down and back live. It checks the uptime and downtime of yo …

Uptime Widget vs Site Uptime Monitoring