UptimeRobot - Free Website Uptime Monitoring
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Uptime Kuma - A fancy self-hosted monitoring tool.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
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.
UpTower - UpTower is an easy-use tool for website monitoring. UpTower checks websites in 1-minute intervals and notifies user via push notifications about every outage immediately. Using UpTower app user can monitor websites and their availability for free.