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.
UptimeRobot - Free Website Uptime Monitoring
Pingbreak - Pingbreak is an original and free monitoring service.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.