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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
Better Uptime - We call you when your website goes down
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.