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.

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

Cacti - Cacti is a complete network graphing solution. It provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features.

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.

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

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