Prometheus - An open-source systems monitoring and alerting toolkit.
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.
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.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
StatsD - Simple daemon for easy stats aggregation.