WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
UptimeRobot - Free Website Uptime Monitoring
Checkify - Super minimal, simple and inexpensive 24/7 uptime monitoring
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PingPing - A simple tool to monitor your website and SSL
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.