The NET-SNMP (formerly UCD-SNMP) package contains various tools relating to the Simple Network Management Protocol including an extensible agent, an SNMP library, tools to request or set information from SNMP agents, tools to generate and handle SNMP traps, a version of the unix 'netstat' command using SNMP and a Tk/perl mib browser. It was originally based on the Carnegie Mellon University SNMP implementation (version 184.108.40.206), but has been greatly enhanced, ported and fixed and barely resembles the original package anymore.
Angel is a simple yet useful tool to monitor the services on your network. Technically speaking, it's a Perl program that runs every 'n' minutes (usually fired from your cron) and calls different perl subprograms (referred to as "plugins" from now on) to do the actual testing. It will then generate an HTML table containing the status of your network.
Network Time Tools (NTT) is a set of network tools designed to provide monitoring of a network and the services on that network, and provide various reports on the hosts/services and optional alerts via email, pager, and cellphones. It comes with a protocol scanner, a bandwidth measurement tool, and a CGI front end that can handle a regular browser or WAP-enabled cellphones via WML.
The Spoofaudit network auditing tool will help you to determine what basic spoofing filters (rfc2827 & rfc3013) are present between two test points on two networks, and what anti-spoofing filters are missing. The tools are designed to work between endpoints that would not normally have filtering between them except anti-spoofing filters.
serverizor allows you to take a normal command- based program, and run it as a tcp server in the background. This means that you can start up such programs once, and then connect to them with a command-line client, and run commands, disconnect, connect again from some other machine, etc. So, you can use it to boot up a large program such as emacs or lisp (in interpretor mode, not in editor/windowing mode) in the background, and then whenever you want to run emacs/lispy scripts, you could just run a serverizor client with a few commands. Therefore, the client and the server are the same program.