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.

siteimpulse

Super Monitoring is an external web application for monitoring website uptime and its basic functions. 
This plugin integrates Super Monitoring interface into WordPress administration panel so you don’t have to log in to supermonitoring.com separately to see your reports or update settings.
Super Monitoring features 
– checking your website every minute 
– detecting different kinds of failures 
– using a worldwide network of monitoring stations to avoid false positives 
– measuring response times 
– content checking 
– web form testing 
– file integrity monitoring 
– instant email & mobile text (SMS) alerts 
– unlimited event history 
– API.
In order to use the plugin you need an account at <a href="https://www.supermonitoring.com/" title="website monitoring" rel="nofollow ugc">www.supermonitoring.com</a>. 
You can sign up for a free trial <a href="https://www.supermonitoring.com/sign-up" title="sign up for a free trial account" rel="nofollow ugc">here</a>.

Super Monitoring

Monitor your website uptime and basic functions with www.supermonitoring.com and access your reports and settings directly in your WordPress panel.

Uptime Widget vs Super Monitoring