k6

k6 is load testing that developers actually enjoy writing. JavaScript test scripts, not XML configs. Built-in protocols for HTTP, WebSocket, gRPC, and browser testing. Run locally, in CI, or scale to millions of requests with Grafana Cloud. It's the load testing tool that doesn't feel like punishment.

If you need to verify your API can handle traffic before launch, k6 gets you there in an afternoon. JMeter is the Java dinosaur — powerful but miserable to use. Locust uses Python and is great for custom scenarios. Artillery is another JS option but less performant. Gatling uses Scala and is enterprise-heavy.

Best for developers who want to add load testing to their CI pipeline without learning a new ecosystem. The JavaScript API means your frontend devs can write performance tests too.

The catch: it's AGPL-3.0, which matters if you're embedding it in a product (not for running tests). The browser testing feature is newer and less mature. And serious distributed load testing requires Grafana Cloud k6 (paid) or setting up your own distributed runners. The free CLI is single-machine only.