Nmap ("Network Mapper") is a free and open source utility for network exploration or security auditing. (GPL)
Cross-platform
SNORT is an open source network intrusion prevention and detection system. (GPL) (GPL)
Cross-platform
Wireshark is the world's foremost network protocol analyzer, and is the de facto (and often de jure) standard across many industries and educational institutions. (GPL)
Linux - FreeBSD - Windows - macOS
3 applications