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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Prometheus - An open-source systems monitoring and alerting toolkit.
Observium - Observium
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Icinga - Icinga is a fork of Nagios and is backward compatible.