Popular free Ruby gem for running tests in parallel on a single machine. Creates isolated environments per process, collects results, and produces a single test report. Works well locally; splits by historical timings but can still leave workers idle when one test is much slower than the rest. Gotchas: setup time multiplies (one DB per worker; Docker image cloning helps), separation isn't always complete (shared caches, filesystem), and single giant tests can't be subdivided.