UptimeRobot - Free Website Uptime Monitoring
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
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).
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!