Prometheus - An open-source systems monitoring and alerting toolkit.
PagerDuty - Cloud based monitoring service
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
VictorOps - We make on-call suck less & help teams to solve problems faster.
StatsD - Simple daemon for easy stats aggregation.
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.