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.
Fluentd - Fluentd is a cross platform open source data collection solution originally developed at Treasure Data.
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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Splunk - Splunk's operational intelligence platform helps unearth intelligent insights from machine data.
Prometheus - An open-source systems monitoring and alerting toolkit.