FrazierWall Linux is a single floppy distribution Linux firewall that is based on the Linux Router Project. It is designed for use with ethernet base internet connections (such as cable modems or xDSL lines). It allows you to share such a connection with several other computers on a LAN. It is easy to setup and maintain, and is available in a Linux configurable software version.
ipcalc takes an IP address and netmask and calculates the resulting broadcast, network, Cisco wildcard mask, and host range. By giving a second netmask, you can design sub- and supernetworks. It is also intended to be a teaching tool and presents the results as easy-to-understand binary values.
Jetmon is a client/server NOC monitor tool, allowing light-weight applet in any browser to monitor network device reachablity information from a central server. It provides simple red-light/green-light indicators in a tiered display, with optional history and status windows, accumulative stats, and exteral URL links to MRTG-like pages, all from a single interface.
The LARTC-HOWTO describes many advanced ways of routing and shaping traffic. Using the HOWTO, you will be able to shape traffic in myriad ways, which will help you lower latency for interactive use when others are downloading or uploading. It will also teach you how to limit certain hosts/protocols' bandwidth usage. It also explains many aspects of routing using the 'ip' tool from the iproute2 suite.
Net::Z3950::SimpleServer is a Perl module which implements the server side of the Z39.50 (information retrieval) protocol. It hides the complexity of network exchanges, packet serialization, and session handling. You are required only to implement simple callbacks to support searching and record retrieval. It is the basis of the "Zoogle" project, which is a Z39.50 gateway to the Google web index.
Netcat is a simple Unix utility which reads and writes data across network connections, using TCP or UDP protocol. It is designed to be a reliable backend tool that can be used directly or easily driven by other programs time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities.