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
Uptime Kuma - A fancy self-hosted monitoring tool.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.