Locust

Define user behavior as Python code, point Locust at your endpoint, and scale up to millions of simulated users. The web UI shows real-time results: requests per second, response times, failure rates.

The reason developers love this: it's just Python. No special syntax, no config files, no learning a testing DSL. If you can write a Python function, you can write a load test. Need to test a complex multi-step flow with authentication? It's just Python code.

MIT. Fully free. No paid tier, no cloud service. The homepage links to PyPI, not a commercial product.

The catch: Locust is CPU-bound because of Python's GIL. For high-throughput testing (10K+ requests/sec), you need to run distributed across multiple workers. Tools like k6 (Go-based) handle higher throughput per instance. Also, the web UI is functional but basic. If you want fancy dashboards, you'll need to export metrics to Grafana.