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Checkmk
Nagios-fork modernized — popular in European DevOps, Raw is free.
Free tier: Raw Edition (open source)
What it does well
- Automatic service discovery simplifies adding and configuring new hosts for monitoring.
- The free 'Raw Edition' is a full-featured, open-source monitoring system without artificial limits.
- Integrates with over 2,000 plugins, covering a wide range of hardware and software.
Where it falls short
- Deployment is self-hosted only, with no official cloud-based (SaaS) option available.
- The user interface and configuration can be complex for beginners, despite improvements over Nagios.
- Advanced features and enterprise support are locked behind expensive, per-host paid tiers.
Alternatives to Checkmk
Nagios
freemium20-year-old infrastructure monitoring — Core is free, XI is paid enterprise.
Zabbix
freeEnterprise-grade open-source monitoring — free forever, paid support available.
Monit
freeClassic Unix process + service monitor — runs locally, restarts services if they die.
CNCF metrics + alerting standard — Blackbox exporter does HTTP/TCP/ICMP probes.