Zipkin

Zipkin shows you exactly where time is spent when a request crosses multiple services. It's a distributed tracing system: each service reports timing data, and Zipkin stitches it into a visual timeline showing the full request journey. When something is slow, you see which service is the bottleneck.

Fully free under Apache 2.0. It's been around since 2012 (originally from Twitter) and is battle-tested at enormous scale. Storage backends include in-memory, MySQL, Cassandra, and Elasticsearch. The UI is simple but functional: search traces by service, duration, or tags.

The catch: Zipkin is traces-only. No metrics, no logs, no dashboards. Modern observability stacks (Grafana + Tempo, SigNoz, Uptrace) bundle all three. If you're starting fresh, Jaeger (also CNCF, also free) has a more modern architecture and better Kubernetes integration. Zipkin's advantage is maturity and simplicity: it does one thing and has been doing it for over a decade.