Zabbix - Track, record, alert and visualize performance and availability of IT resources
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
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.
SolarWinds NPM - Advanced network monitoring for on-premises, hybrid, and cloud. Download a FREE 30-day trial of SolarWinds Network Performance Monitor (NPM) today!
Munin - PnP networked resource monitoring tool that can help to answer the what just happened to kill our performance