Prometheus - An open-source systems monitoring and alerting toolkit.
SevOne - SevOne provides network managers with monitoring, troubleshooting, and performance reporting capabilities.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
SolarWinds Network Bandwidth Analyzer - Download a 30-day free trial of Network Bandwidth Analyzer Pack, easy-to-use software that delivers real-time monitoring, alerting, and reporting for routers, switches, and other SNMP-enabled devices.
StatsD - Simple daemon for easy stats aggregation.
Obkio - Simple Network Monitoring and Network Troubleshooting software to empower users to identify and troubleshoot network & application slowdown from the end-user perspective.