UptimeRobot - Free Website Uptime Monitoring
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
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).
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting