Curler - Dead Simple Free 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.
ncScale - Datadog for No-Code, 1-click Monitoring for No-Code stack!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Better Uptime - We call you when your website goes down