Prometheus - An open-source systems monitoring and alerting toolkit.
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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources