Interesting discussion of FOSS solutions
Here's my $0.02:
I wore the network and monitoring hats as part of an IT team of 15 supporting a university of 2000 students and ~500 faculty and staff. I was *the* network admin/engineer/architect.
Zabbix looked a bit strange, though I didn't invest much time in it.
Spiceworks is 80% terrible at network monitoring. Great for configuration backups on our network, though! Couldn't use it for desktops since we were mostly Macs.
Zenoss is has similar network monitoring functionality to Spiceworks but is 90% better at it. Seemed to have very limited server monitoring IIRC. We used this for a bit to monitor the network in between other solutions. Never got the trap receiver working properly.
Cacti was too limited for our needs, though good at what it did (if ugly).
MRTG was much the same as Cacti.
NAGIOS was, by all accounts, extremely powerful. Never did get it configured though as it all seemed too much of a PITA, Yes, we even tried a couple of wrappers that were supposed to address using the GUI to add monitors, etc.
Xymon was a PITA to configure and maintain though decent at what it did.
PRTG was what we eventually settled on. It was reasonably priced and did both network and server monitoring competently. There was a lot to learn but the system could be configured in a basic way with advanced configuration applied as time allowed for learning. It was not the best system available, and I wouldn't sing its praises from the hilltops, but it's probably worth checking out even if you hate proprietary solutions. It is reasonably competent, with alerts based on groups, dependency monitoring (router goes down and stop monitoring all the downstream switches and do NOT alert me again), SSL expiry monitors, frequent updates which occasionally add real features (and they fixed several SSL vulns in a matter of a few months), etc.