StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UptimeRobot - Free Website Uptime Monitoring
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.