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.
UptimeRobot - Free Website Uptime Monitoring
Pingbreak - Pingbreak is an original and free monitoring service.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
DownDetector - A simple service to tell you whether a website is down.