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.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
SolarWinds NPM - Advanced network monitoring for on-premises, hybrid, and cloud. Download a FREE 30-day trial of SolarWinds Network Performance Monitor (NPM) today!
PagerDuty - Cloud based monitoring service