The best replacement for zenoss is nagios.

The best alternatives to zenoss are rated as following:
★★★★★  nagios
★★★  zabbix
★★★  opennms
★★  splunk
★★  spiceworks
★  opsview
★  ganglia
★  pandora fms
★  cacti
★  prtg
★  groundwork
★  solarwinds
★  nimsoft
  hyperic
  munin
  centreon
  scom

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Speaking about zenoss, one probably shouldn't forget about nagios plugins.

Other search terms related to zenoss and their ratings are:
★★★★★ nagios plugins
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Google Trends

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