Uptime Kuma - A fancy self-hosted monitoring tool.
UptimeRobot - Free Website 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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Statping - Status Page for monitoring your websites and applications with beautiful graphs, analytics, and plugins. Run on any type of environment.
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.