LOGBack - Logging framework
Fluentd - Fluentd is a cross platform open source data collection solution originally developed at Treasure Data.
Serilog - Backend Development and Utilities
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.
log4net - log4net is a tool to help the programmer output log statements to a variety of output targets.
Splunk - Splunk's operational intelligence platform helps unearth intelligent insights from machine data.