rsync is a replacement for rcp (and scp) that has many more features. It uses the "rsync algorithm" which provides a very fast method for remote files into sync. It does this by sending just the differences in the files across the link, without requiring that both sets of files are present at one of the ends of the link beforehand.
MailScanner is an email virus scanner, vulnerability protector, and spam tagger. It supports the Postfix, Sendmail, Exim, Qmail, and ZMailer MTAs, and the Sophos, McAfee, F-Prot, F-Secure, CommandAV, InoculateIT, Inoculan, eTrust, Kaspersky, Nod32, AntiVir, BitDefender, RAV, Panda, DrWeb, ClamAV, and other anti-virus scanners. It uses SpamAssassin for highly successful spam identification, and is designed to handle denial of service attacks. It will detect password-protected zip files and apply filename checking to their contents. It is very easy to install, requires no changes at all to your sendmail.cf file, is designed to be lightweight, and won't grind your mail system to a halt with its load. It can be integrated into any email system, regardless of the software in use.
Performance Co-Pilot (PCP) is a framework and set of services for supporting system-level performance monitoring and performance management. It provides a unifying abstraction for all of the interesting performance data in a system, and allows client applications to easily retrieve and process any subset of that data using a single API. A client-server architecture allows multiple clients to monitor the same host, and a single client to monitor multiple hosts. Archive logging and replay are integrated so that a client application can use the same API to process real-time data from a host or historical data from an archive.
Ganglia is a scalable distributed monitoring system for high-performance computing systems such as clusters and grids. It is based on a hierarchical design targeted at federations of clusters. Ganglia is currently in use on over 500 clusters around the world and has scaled to handle clusters with 2000 nodes.
Munin is a system to gather and graph all sorts of information. You can install a node on the various machines in your network, as well as on a central server. The nodes will know how to extract various kinds of information, such as load average and bandwidth usage, and will wait for the server to request these values. The output is in HTML format. The server can optionally send notifications if any of the values move outside of a specified range (and when they move back into it).
SendIP is a command-line tool to send arbitrary IP packets. It has a large number of options to specify the content of every header of a RIP, RIPng, BGP, TCP, UDP, ICMP, or raw IPv4/IPv6 packet. It also allows any data to be added to the packet. Checksums can be calculated automatically, but if you wish to send out wrong checksums, that is supported too.