UptimeRobot - Free Website Uptime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Uptime Kuma - A fancy self-hosted monitoring tool.
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.
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring