Nagios - Complete monitoring and alerting for servers, switches, applications, and services
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
Linux Dash - A drop-in, low-overhead monitoring web dashboard for a Linux machine.
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.