This is an uncomplete list of freeware Unix programs I have used for about ten years and that I continue to recommend with the same entusiasm I had when I installed them for the first time on Sun Solaris and SunOs machines.
Perl my preferred programming and scripting language
Perl is designed to make the easy jobs easy, without making the hard jobs impossible. Perl is a language for powerful text processing, filesystem manipulation, process management, database administration, client-server programming, secure programming, web-based information management and even for object-oriented programming; it was designed to be a glue language from the start. After learning it, back in 1992, i stopped to write small C programs, non trivial shell scripts, using awk and sed. Almost everything needed by a system administrator can be done very fast and with fun using Perl.
Amanda the perfect network backup solution
It is a very good network backup solution: you have a backup server workstation with attached a tape, many client workstations (also PC's, but in this case you need samba); you decide how many tape to use, the host and disk list to backup, how frequently to do a full backup of each disk and amanda does the rest. The software every night does an incremental backup of each disk in the network and a full backup of some disks to satisfy your policies.
Swatch the syslog watcher
Swatch is designed to monitor system activity. Swatch requires a configuration file which contains patterns to look for and actions to do when each pattern is found. I have redirected the syslogd output of every unix workstation to a single logging workstation, there swatch monitor the syslogd output and alert me, via e-mail, when events, like disk full etc., happens everywhere in the network.
Sysinfo shows system information
Sysinfo is a program which shows various pieces of information about the hardware and operating system software configuration of the host it's run on. Very useful: I have a file with its output for every workstation. When some workstation dies i can always call technical service and tell them exactly how it was configured: its processor, its disks, its RAM etc.