WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
Suprvise is a real-time health monitoring and errors tracking dashboard.
UptimeRobot - Free Website Uptime Monitoring
Monitority - Simple website monitoring service (sms/email alerts)
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
Oh Dear! - Website monitoring, without the hassle