Platypus - Realtime server monitoring and reporting
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.
PagerDuty - Cloud based monitoring service
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Monitornator - Server monitoring made simple
Prometheus - An open-source systems monitoring and alerting toolkit.