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