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.
Endian Firewall is an all-in-on Linux security distribution that turns any system into a full-featured security appliance. It features a stateful packet inspection firewall, application-level proxies for various protocols (HTTP, POP3, SMTP), anti-virus support, virus and spam filtering for email traffic (POP and SMTP), content filtering of Web traffic, and a "hassle free" VPN system based on OpenVPN.
Saswire is a Perl program which generates a database in SDBM format containing the modification time, MD5 checksum, size, and creation time for a list of files specified in a input file. It is designed to check for unwanted modifications of Unix system binaries if security is compromised.
Complete System Resource Monitor and Task Organizor consists of a daemon, a client, and a WWW-server (within the daemon). The daemon can run tasks and handle client-daemon and WWW-daemon requests. Clients can receive statistics and issue commands. Monitoring and maintenance is performed as prescheduled tasks. The system includes a process monitor and handler, a basic WWW-client, and a console client.