TCPDump, a powerful command-line packet analyzer; and libpcap, a portable C/C++ library for network traffic capture. It prints out a description of the contents of packets on a network interface that match the boolean expression. The Packet Capture library provides a high level interface to packet capture systems. All packets on the network, even those destined for other hosts, are accessible through this mechanism.   

TCPDump -  Network Packet Analyzer

Snort is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Snort can perform protocol analysis and content searching/matching. It can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.
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Snort has a real-time alerting capability as well, incorporating alerting mechanisms for syslog, a user specified file, a UNIX socket, or WinPopup messages to Windows clients. Snort has three primary uses: a straight packet sniffer like tcpdump, a packet logger (useful for network traffic debugging, etc), or a full-blown network intrusion prevention system.

Snort is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Snort can perform protocol analysis and content searching/matching. It can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.

Snort - Network Intrusion Prevention and Detection System

Robust, Lightweight, Cross-platform, Template Software. STLSoft is a collection of STL and standard library extensions, and facades over operating-system and 3rd-party APIs.

STLSoft - C and C++ Libraries

Open vSwitch is well suited to function as a virtual switch in VM environments. In addition to exposing standard control and visibility interfaces to the virtual networking layer, it was designed to support distribution across multiple physical servers. Open vSwitch supports multiple Linux-based virtualization technologies including Xen/XenServer, KVM, and VirtualBox.

Open vSwitch - Multilayer Virtual Switch

IPFire is a server distribution with intended to use as a firewall. It focuses on flexibility, and scales from small to middle sized business networks and home networks. Beginning with a small firewall system of a few megabytes, it is possible to run IPFire as a file server or VPN gateway for staff, branches or customers. This modularity means that yor version of IPFire runs with exactly what you require and nothing more.

IPFire - An open source firewall solution

SmoothWall is a open source Firewall distribution. It Supports LAN, DMZ, and Wireless networks, Static Ethernet, DHCP Ethernet, PPPoE, PPPoA using various USB and PCI DSL modems. It provides traffic stats, POP3 email proxy with Anti-Virus, Web proxy for accelerated browsing and lot more.

SmoothWall - Express Open Source Firewall Project

IPCop provides secure and stable Linux Firewall Distribution. It is a complete Linux Distribution whose sole purpose is to protect the networks it is installed on. It is geared towards home and SOHO users. The IPCop web-interface is very user-friendly and makes usage easy.

IPCop - Linux firewall distribution

ScanSSH supports scanning a list of addresses and networks for open proxies, SSH protocol servers, Web and SMTP servers. Where possible ScanSSH, displays the version number of the running services. ScanSSH protocol scanner supports random selection of IP addresses from large network ranges and is useful for gathering statistics on the deployment of SSH protocol servers in a company or the Internet as whole. 

ScanSSH -  Scans the network for open proxies

ZMap is an open-source network scanner that enables researchers to easily perform Internet-wide network studies. With a single machine and a well provisioned network uplink, ZMap is capable of performing a complete scan of the IPv4 address space in under 45 minutes, approaching the theoretical limit of gigabit Ethernet. ZMap can be used to study protocol adoption over time, monitor service availability, and help us better understand large systems distributed across the Internet.

Zmap - The Internet Scanner

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TCPDump - Network Packet Analyzer