The SINUS Firewall is a TCP/IP packet filter for Linux. Some of its features are stateful inspection of TCP communications, text-based configuration, graphical management interface for configuration of several firewalls, dynamic rules, prevention of packet and address spoofing, extensive logging, alerting, and counter intelligence.
SING stands for 'Send ICMP Nasty Garbage.' It is a tool that sends ICMP packets fully customized from command line. Its main purpose is to replace and complement the ping command, adding certain enhancements as fragmentation; send and receive spoofed packets; send many ICMP information types (echo as the old ping, address mask, timestamp, and router discovery) and errors (redirect, unreach, and time exceeded); and send monster packets. It also supports loose and strict source routing and record routing.
SmoothWall Express is a network firewall, designed with home and small business users in mind. It is based upon a security-hardened subset of the GNU/Linux operating system and is completely free to use, download, and distribute. SmoothWall Express offers facilities and features normally only seen in expensive commercial offerings.
tcpxd is a TCP/IP relay or proxy, allowing a connection to a port on a system to be forwarded to another port on any other system. It is useful for firewalls and service relocations, and is small, simple, and fast. It's also easier to learn than netcat and supports TCP_NODELAY, alternate local ports, partial connection closures, and non-blocking connects.
tircproxy is a full featured IRC proxy. It can proxy DCC CHAT, DCC SEND and other DCC requests transparently to the user, block common trojans (such as script.ini), hide the user's identity and more. It can be used with the Linux kernel or IPF transparent proxy features to transparently proxy IRC sessions.
TrinityOS is a step-by-step, example-driven HOWTO on building a very functional Linux box with strong security in mind. TrinityOS is well known for its strong packet firewall ruleset, Chrooted and Split DNS (v9 and v8), secured Sendmail (8.x), Linux PPTP, Serial consoles and Reverse TELNET, DHCPd, SSHd, UPSes, system performance tuning, the automated TrinityOS-Security implementation scripts, and much more.