I would love to get nagios going on my unraid box though I'm unsure what would be the best way to go about it. I am in the process of setting up a nagios server so once it is set up I will look into getting it run on unraid. I'm not even sure how to get most of the advanced stats like current bandwidth and connected users displayed locally let alone get them exported across the network though.
Also Billk, I believe that Nmap more intend for initial discovering of network devices rather than a constant monitor of them. This is where nagios could be used, in fact the guys over at the official forum are trying to combine the two programs (nagios + nmap). http://www.meulie.net/portal_plugins/forum/forum_viewtopic.php?11185.0#post_11252