eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources
UptimeRobot - Free Website Uptime Monitoring
Eureka - Eureka is a contact center and enterprise performance through speech analytics that immediately reveals insights from automated analysis of communications including calls, chat, email, texts, social media, surveys and more.
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.