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.

Prometheus - An open-source systems monitoring and alerting toolkit.

systemd - systemd is a replacement for the init daemon for Linux (either System V or BSD-style).

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

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