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
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.
Observium - Observium
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Prometheus - An open-source systems monitoring and alerting toolkit.