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.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases
Prometheus - An open-source systems monitoring and alerting toolkit.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
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.