Just a quick note to say that I have been setting Nagios up over the past few days on my Debian box. If you have been considering installing some kind of monitoring software, I can highly recommend it - it's well worth the time it takes to configure!
So far, I have set up Nagios to automagically ping all my managed sitches and my main servers to check that they are still 'alive' and I am currently setting up pings to my network printers, along with pulling the JetDirect printer status.
I will be configuring further devices and hopefully adding in some SNMP stuff over the next week or two. At the moment it is proving very handy to be able to see at a glance which printers are low on toner or which network links are down.