Suprvise is a real-time health monitoring and errors tracking dashboard.
UptimeRobot - Free Website Uptime Monitoring
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
Monitority - Simple website monitoring service (sms/email alerts)
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
Oh Dear! - Website monitoring, without the hassle