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.
Pingbreak - Pingbreak is an original and free monitoring service.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
DownDetector - A simple service to tell you whether a website is down.
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.