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