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.
UptimeRobot - Free Website Uptime Monitoring
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
Uptime Kuma - A fancy self-hosted monitoring tool.
Pingbreak - Pingbreak is an original and free monitoring service.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).