Argus is a system and network monitoring application. It will monitor nearly anything you ask it to monitor (TCP + UDP applications, IP connectivity, SNMP OIDS, etc). It presents a clean, easy-to-view Web interface. It can send alerts numerous ways (such as via pager) and can automatically escalate if someone falls asleep.
Chronic is a constraints-based opportunistic application-level scheduler. Unlike cron, which schedules according to a time specification, it schedules tasks when specified constraints are met. In other words, it schedules during a window of opportunity when the circumstances are favorable.
Dummynet is a flexible tool originally designed for testing networking protocols, and since then (mis) used for bandwidth management. It simulates/enforces queue and bandwidth limitations, delays, packet losses, and multipath effects. It also implements a variant of Weighted Fair Queueing called WF2Q+. It can be used on user's workstations, or on FreeBSD machines acting as routers or bridges.
IOG is a network I/O byte grapher made to graph cumulative KB/MB/GB totals for hours/days and months. It is intended to be simple, fast (support thousands of interfaces) and integrate well with MRTG (not required). Data for each host is updated hourly and HTML graphs are created. It uses a data consolidation algorithm which allows for a small, non-growing database file for each host. Perl is required, but no external graphing libraries or binaries are needed.
l4ip is a daemon that extends IPFilter's simple round-robin L4 load balancing with health checks. It takes care of monitoring and dynamically adding and removing ipnat rules in the kernel. It fully supports TCP, UDP, and system()-style executions for health checks, including send/expect scripts, SSL connections, and much more.
Openwsman provides an implementation of the Web Services Management specification and expose system management information on the Linux operating system using the WS-Management protocol. It is based on a suite of Web services specifications and usage requirements that exposes a set of operations focused on and covering all system management aspects.
pv (Pipe Viewer) is a terminal-based tool for monitoring the progress of data through a pipeline. It can be inserted into any normal pipeline between two processes to give a visual indication of how quickly data is passing through, how long it has taken, how near to completion it is, and an estimate of how long it will be until completion.