Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
Curler - Dead Simple Free Uptime Monitoring
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.
Better Uptime - We call you when your website goes down