Netdata - Monitor your servers, containers, and applications, in high-resolution and in real-time.
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.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases
Zabbix - Track, record, alert and visualize performance and availability of IT resources
eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services