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