UptimeRobot - Free Website Uptime Monitoring
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.
Pingbreak - Pingbreak is an original and free monitoring service.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!