ERALION

Monitoscope is a webservice that monitors websites and alerts in case of access failure, it allows up to monitor 1 free website.

Monitoscope

Get alerted when your website go down with Monitoscope.

GTmetrix

GTmetrix has created GTmetrix for WordPress – a WordPress plugin that actively keeps track of your WP install and sends you alerts if your site falls below certain criteria.
Run analyses, schedule reports on a daily, weekly or monthly basis, and receive alerts about the status of your site all from within your WordPress Admin!

GTmetrix for WordPress

Plugin isn't showing the toolbar link in the latest version of WP now :(

Fantastic plugin but...

GTmetrix can help you develop a faster, more efficient, and all-around improved website experience for your users. Your users will love you for it.

Monitoscope vs GTmetrix for WordPress