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
PingTurk - PingTurk is a cloud-based platform to monitor website from many locations and behind different networks, reporting on customers experience.
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
AlertBot - AlertBot provides availability and performance monitoring from an end user perspective.