The best replacement for nagios is zabbix.

The best alternatives to nagios are rated as following:
★★★★★  zabbix
★★★★★  zenoss
★★★★  cacti
★★★  opennms
★★  icinga
★★  solarwinds
★  spiceworks
★  new relic
★  sensu
★  opsview
★  hyperic
★  munin
★  prtg
★  scom
★  big brother
★  graphite
★  cloudwatch
★  monit
  whatsup gold
  tivoli
  openview
  splunk
  hobbit
  ganglia
  xymon
  sitescope
  observium
  ipmonitor
  mrtg
  nagios
  check_mk
  whatsup
  groundwork

Your alternative for nagios is not listed? Suggest!
Speaking about nagios, one probably shouldn't forget about cacti.

Other search terms related to nagios and their ratings are:
★★★★★ cacti
★★★★ nrpe
★★★ snmp
★★★ mrtg
★★★ graphite
★★ splunk
★★ otrs
★ netflow
You have in mind a companion for nagios and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of nagios in all years. Click on graph for more.
nagios
zabbix
zenoss