Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Suprvise - Know instantly when your sites are down.
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.
Oh Dear! - Website monitoring, without the hassle
UptimeRobot - Free Website Uptime Monitoring