Nagios is a network monitoring software package, which allows full monitoring, alerting, and trending of a myriad of data relating to systems on a network.
Full information can be found on the Project Homepage. Here I will present a few of my personal notes.
etc configuration files
For my personal use, I keep all top-level configuration files in the etc directory. My main nagios.cfg uses directory includes. I have a directory called "hosts" under etc, which is included as a whole. Each host has its own .cfg file in this directory, so that a host can be easily edited and updated.