Every admin have it’s own collection, here is mine with short description:
slurm - produces graph of RX and TX speed of interace
pfqueue - ncurses based program to manage deffered, active, incoming queue of postfix
xtail - can monitor as many directories and logs as You want
colorize - it can be attatched to any other command via pipe, I use it mostly with `xtail /var/log | colorize`
pkg_tarup - I'm using in mainly to create set of actually working packages in case of update failure
smartmontools - it is not so small and do many things but it does exactly what i need
uptimed - collects and sends mails of best uptimes and os versions
mercurial - very very very fast sync of pkgsrc
I will add some tools when I find something new or remember something i forgot :)