LibreNMS

LibreNMS is a community-developed, fully open-source network monitoring system, which makes it the zero-licence-cost option in this category. There is no paid tier and no vendor to negotiate with: the trade is that you host, configure and maintain it yourself.

Mapping is native rather than an add-on. It ships an availability map, a dependency map, a general network map, custom maps and a world map, built on top of auto-discovery that finds devices and their relationships over SNMP. Around that sits a substantial alerting system with rules, templates and a long list of delivery transports covering Discord, Slack, Alertmanager, Cisco Spark, Elasticsearch and many more. It scales through distributed polling, a dispatcher service and options such as RRDCached and Redis, and exposes a full API for automation. Storage can feed InfluxDB, Prometheus, OpenTSDB or Kafka. Being community software, support comes from documentation and the community rather than a vendor contract.