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.
Prometheus - An open-source systems monitoring and alerting toolkit.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
StatsD - Simple daemon for easy stats aggregation.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Hosted Graphite - Graphite as a service