Zabbix - Track, record, alert and visualize performance and availability of IT resources
Observium - Observium
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.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Icinga - Icinga is a fork of Nagios and is backward compatible.
LogicMonitor - LogicMonitor is the SaaS performance monitoring platform for the world's best IT teams. Deploy Fast, Monitor More, Improve Ops.