Open source Network scanners

Angry IP Scanner

Angry IP Scanner is an open-source and cross-platform network scanner designed to be fast and simple to use. It scans IP addresses and ports as well as has many other features. (GPL)

Linux - Windows - MAC OS X

Nmap

Nmap ("Network Mapper") is a free and open source utility for network exploration or security auditing. (GPL)

Cross-platform

OSSIM

OSSIM, or the Open Source Security Information Management, is a collection of tools designed to aid network administrators in computer security, intrusion detection and prevention. (GPL)

Linux

SNORT

SNORT is an open source network intrusion prevention and detection system. (GPL) (GPL)

Cross-platform

Wireshark

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 - MAC OS X

5 applications