BLËSK - BLËSK is a suite of applications for network monitoring and management.
SolarWinds Log Analyzer - SolarWinds Log Analyzer tool provides comprehensive log management and analysis with out-of-the-box visibility into performance and availability. Free trial!
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
MRTG - MRTG (or Multi Router Traffic Grapher) is an open-source network monitoring tool.
Splunk - Splunk's operational intelligence platform helps unearth intelligent insights from machine data.