This is a very coarse survey in early 2013. I am looking for very simple server monitor, watching hard disk / memory usage, running of process, etc., for free. I'm just putting some first opinion and this is in no way a serious assessment.
http://sixrevisions.com/tools/10-free-server-network-monitoring-tools-that-kick-ass/
http://en.wikipedia.org/wiki/Comparison_of_network_monitoring_systems
(copy the matrix into a spreadsheet, analysis, as of early 2013)
Total: 51 products
Non-commercial license: about 25
Non-commercial and distributed: 22
Monit: not free
Ganglia: seems to be more complex than I wish and be targeting clusters, etc.
Munin: worthwhile further check up; written in perl but plugin can be in any language, at least so claimed by their wiki; it's said to be easy to extend
Cacti: according to some description, it seems to be a munin with better data viewing capability
Nagios: mentioned to be complex but powerful; only core is free, other modules are charged; a demo for the core seems to be quite good - impressed
Zabbix: very impressing website, GPL license, enterprise support (developed by an Eastern European company); screenshots show very sophisticated system; commercial users;
Observium: auto-discovering. freeware. demo looks great! quite some big name / telecom users using it; its wiki says this one does not seek to replace other monitoring tools but aims to be a complement; so it is often deployed with Nagios
Zenoss: written in python, accept Nagios plugin; does not seem to be free.
Collectd: mentioned to be fast, low resource usage, but seems to work on just local system.
Argus: seem to be a "one man project" but still impressing. Just not sure about the future of this project.