Open Source Network Monitoring
You cannot fix what you cannot see, but most commercial monitoring prices visibility by the device, sensor, or node - so the bigger and more complex your network gets, the more it costs to watch exactly when blind spots hurt most. The open source network monitoring here polls and traps the same switches, hosts, and services without charging per endpoint, so you can instrument everything you run and keep the metrics and alerting logic under your own roof.
Uptime Kuma
Self-hosted uptime monitoring with status pages, alerts, and a browser-based dashboard
Netdata
Sees every metric every second, then uses per-metric ML to flag what looks abnormal
Zeek
Network security monitoring framework for high-volume application-layer traffic analysis
MeshCentral
Self-hosted web-based remote monitoring and management for computers on local networks or the Internet
Suricata
Network IDS, IPS, and NSM engine for monitoring and protecting traffic
pfSense CE
FreeBSD-based firewall and router distribution managed entirely from a web interface
LibreNMS
Autodiscovering SNMP network monitor that fingerprints your gear and graphs it automatically
OPNsense
FreeBSD-based firewall and routing platform with a web GUI, VPN, IDS/IPS, and high availability
Snort 3
Network intrusion detection and prevention with scriptable config and threaded packet processing