M/Monit - Monit is a free open source utility for managing and monitoring, processes, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
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.
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Better Uptime - We call you when your website goes down