Linux-HA aka Heartbeat is a modular package to control high-availability clustering. In spite of it's name it is not limited to Linux (although that is the primary platform), It has an automake based compile and has been used on *BSD, Solaris, and to some extent on AIX as well.

It can hearbeat in multiple ways (UDP broadcast, multicast, and unicast as well as over serial ports, although the serial port heartbeat has been accidently broken in some versions), and over multiple channels (up to 32 as of the time of writing)

It can support sub-second failover

It delays heartbeat checking on initial boot to allow switches time to get through their spanning tree detection timeouts.

Often Unix system administrators need an easy way to redirect any stderr output generated from crontab entries to a specific email address. With the ifinput binary, it's simple. For example:

20 8 * * * /usr/local/sbin/daily_stuff.sh 2>&1 | /usr/local/bin/ifinput mailx -s "Error: daily_stuff.sh" system-errors@foo.org

Just arrange the crontab entries so that the periodic jobs don't generate stdout unless there's something interesting there. Then redirect stderr to stdout and pipe both to ifinput.

ifinput can be used in a variety of different scenarios, not just cron jobs.
The source code is available here: http://www.generalconcepts.com/resources/software/gctools-1.0.tar.gz
It has a very small footprint and will compile without issue on most unix systems.

Bcfg2 is a generative configuration management system that can be used for management of aspects of client systems, complete control of client systems, and the range in between. Bcfg2 is designed to work well in pre-existing environments, and in the presence of manual administration.
Bcfg2's most interesting feature is a web frontend that presents client configuration conformance information to administrators. This provides administrators with a direct way to observe how good of a job they are doing describing how their network should look. To our knowledge, this approach is unique to bcfg2, both as a goal and as a supported operation.

I like to use this to throw together nice web frontends for the PHBs to look at stats & stuff. It's pretty simple to use, interfaces easily to a database--very useful for the sorts of sysadmin-plus stuff I tend to do.

Nmap is a powerful tool for discovering hosts on a network and enumerating what service they are offering. This can be used to find vulnerable systems, to locate rogue services on your network or simply for a first step in troubleshooting.

daemontools is a collection of tools for managing UNIX services.

supervise monitors a service. It starts the service and restarts the service if it dies. Setting up a new service is easy: all supervise needs is a directory with a run script that runs the service.

multilog saves error messages to one or more logs. It optionally timestamps each line and, for each log, includes or excludes lines matching specified patterns. It automatically rotates logs to limit the amount of disk space used. If the disk fills up, it pauses and tries again, without losing any data.

A less encumbered version is available at http://smarden.org/runit/