Pingbreak - Pingbreak is an original and free monitoring service.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
DownDetector - A simple service to tell you whether a website is down.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
Better Uptime - We call you when your website goes down