Datadog is a monitoring service for cloud-scale applications, bringing together data from servers, databases, tools, and services to present a unified view of…

Graylog helps you can observe any established or rejected network connection, find those that are unusual to tighten rules and mitigate attacker activity.

Scalyr is a server log monitoring tool.

Fluentd is a cross platform open source data collection software.

Nagios Log Server is centralized log management, monitoring & analysis software. Quickly & easily manage, monitor and analyze log data.

Splunk is a software for searching, monitoring, and analyzing machine-generated big data.