nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
SolarWinds Network Configuration Manager - SolarWinds is a leading provider of powerful and affordable IT infrastructure management software. Make IT look easy.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Cisco Network Assistant - Cisco Network Assistant simplifies wired and wireless network management, and provides critical product information using Cisco Active Advisor.