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.
Platypus - Realtime server monitoring and reporting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Monitornator - Server monitoring made simple
Better Uptime - We call you when your website goes down
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.