nxlog - NXLog offers log management solutions for companies of all sizes.
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.
Serilog - Backend Development and Utilities
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.
LOGBack - Logging framework