Zabbix - Track, record, alert and visualize performance and availability of IT resources
Beats - Beats is the platform for single-purpose data shippers that is installed as lightweight agents and send data to machines to Logstash or Elasticsearch.
Observium - Observium
Riemann - Container Monitoring
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Icinga - Icinga is a fork of Nagios and is backward compatible.