Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Observium - Observium
Better Uptime - We call you when your website goes down