Zabbix - Track, record, alert and visualize performance and availability of IT resources

Nagios - Complete monitoring and alerting for servers, switches, applications, and services

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.

Linux Dash - A drop-in, low-overhead monitoring web dashboard for a Linux machine.

Munin - PnP networked resource monitoring tool that can help to answer the what just happened to kill our performance

Netdata - Monitor your servers, containers, and applications, in high-resolution and in real-time.