Nagios: is clearly the easiest to install/administer (even from source). Graphs could be better though but plugins and community support/documentation are probably the best.
Zabbix: is easily the best for graphs but still a slight learning curve and documentation could be better. SNMP support is lacking but is supposedly part of a commercial downstream project and is a project request for 1.8 (next version), slightly worried about agent having security vulnerabilities as well (although it does seem to be patched regularly).
ZenOSS: could be great but still has too many bugs and there's too much of a learning curve. In future I think this will be 'the standard' for larger networks if Zabbix doesn't fix its SNMP capabilities and/or Nagios doesn't fix its graphs or or plugin to do better graphing.
SNIPS: was a disaster. Don't want to even talk about it. Everything is manually configured (crontab, source code install, etc...) guessing it was developed a long time ago and hasn't been worked on since.
OpenNMS: will try later...
All have the ability to Email/SMS provided correct plugin.
All have SNMP/IP capabilities and other service monitoring.
