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.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue
OnlineOrNot - Uptime monitoring for software teams (even teams of one).