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.
qmqtool is a utility designed for viewing and safely modifying the contents of a qmail queue. It is extremely feature rich, with features including finding abusers, sorting messages by sender/sender's IP, showing stats, requeueing messages, queue consistency check, queue backup/restore, and others.
Spong is a simple system-monitoring package. It features client-based monitoring (CPU, disk, processes, logs, etc.), monitoring of network services, grouping of hosts (routers, servers, workstations, and PCs), host-specific contact and downtime information, configuration on a client-by-client basis, results displayed via the Web, history of problems, messaging (via email or pager) when problems occur, and verbose information to help diagnosis problems. It communicates via simple TCP-based messages, and is written in Perl.
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.