PagerDuty - Cloud based monitoring service
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.
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
Prometheus - An open-source systems monitoring and alerting toolkit.
Squadcast - Automate incident response, reduce downtime and enhance your tech teams’ delivery with a unified platform.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services