k6

K6 lets you write those load tests in JavaScript and run them from the command line. No JMeter XML, no clunky GUI. You write a script, run `k6 run script.js`, and get performance metrics back in seconds.

Go, AGPL v3 (important, more on that below). Grafana acquired k6 in 2021. The CLI is fast, resource-efficient, and scriptable. You can test HTTP, WebSocket, gRPC, and browser-level interactions. Results integrate natively with Grafana dashboards, Prometheus, Datadog, and most observability stacks. The CLI is free and always will be. Write tests, run them locally, pipe results wherever you want. No limits on virtual users or test duration when self-hosted.

Grafana Cloud k6 is the paid option: starts at $0 for limited cloud runs (50 cloud tests/month on the free plan), then scales with usage. Cloud gives you distributed load generation from multiple regions, something the local CLI can't do alone. Solo and small teams: the CLI is all you need. Run tests from your CI pipeline, export to Grafana. Medium teams: cloud makes sense when you need to simulate traffic from multiple geographic locations. Large teams: Grafana Cloud k6 or self-host the execution infrastructure.

The catch: the AGPL license means if you modify k6 and offer it as a service, you must open source your changes. For running tests internally this doesn't matter. But if you're building a testing platform on top of k6, talk to a lawyer first.