The best replacement for ganglia is nagios.

The best alternatives to ganglia are rated as following:
★★★★★  nagios
★★★★  nuclei
★★★  ganglion
★★★  cacti
★★  graphite
★★  zabbix
★★  glia
★  nerve
★  cerebellum
★  munin
★  nucleus
★  prevertebral ganglia
★  plexus
★  collectd
  substantia nigra
  sympathetic chain ganglia
  zenoss
  ganglions

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

Other search terms related to ganglia and their ratings are:
★★★★★ synapse
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ganglia
nagios
nuclei