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
DownDetector - A simple service to tell you whether a website is down.
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.