WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
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.
UptimeRobot - Free Website Uptime Monitoring
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue