Netdata is an open-source (GPLv3) real-time server monitoring tool, collecting system metrics at second-level resolution with hundreds of preconfigured alert rules and 200+ application monitoring.
Uptime Kuma is a free, open-source, self-hosted uptime monitoring tool released in 2021 with Docker one-click deployment. Supports 20+ monitor types (HTTP, Ping, Port, SSL), a beautiful status page, and 90+ notification channels.
Riemann is a 2012-released open-source event stream processor built in Clojure for millisecond-level low-latency processing. It supports high-throughput event aggregation, filtering, transformation, and correlation with TCP/UDP multi-protocol ingestion for real-time monitoring event routing.