WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
UptimeRobot - Free Website Uptime Monitoring
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.