Jaeger

It's a distributed tracing platform: it tracks requests as they flow through your services, showing you exactly where time is spent. Service A calls Service B calls Service C, and Jaeger draws the full timeline.

CNCF graduated project, originally built by Uber. It collects traces from your services, stores them, and gives you a UI to search and visualize request flows. Supports OpenTelemetry natively, which means you instrument your code once and can switch tracing backends later.

Apache 2.0. Fully free, no paid tier.

The catch: running Jaeger in production is real ops work. You need a storage backend (Elasticsearch, Cassandra, or Kafka), and those have their own operational costs. The UI is functional but not pretty; teams that want polished dashboards often export traces to Grafana. For small deployments, SigNoz bundles tracing, metrics, and logs in one tool and may be simpler to start with.