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 Kuma - A fancy self-hosted monitoring tool.
UptimeRobot - Free Website Uptime Monitoring
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Cachet - Cachet is a free, open source status page for your API, service or company. Built with all of the features that you'd expect from a status page.