Performant Labs ships the open-source Drupal testing tools the rest of this site is about — ATK and Testor — and is currently building Aftersight. The work below extends that into adjacent areas: a quality-process initiative, a few content / commerce kits, and a payment integration. All maintained on drupal.org under aangel.
Automated Testing Kit (ATK)
A library of tests and helper functions for end-to-end testing of Drupal sites, for both Playwright and Cypress.
Why it exists: every Drupal site re-writes the same twenty login / entity-reference / permissions tests in each new project. ATK ships them pre-written, together with the helper functions needed to make them reliable against Drupal's specific quirks. Maintained on drupal.org as a supported module, including a Drupal 7 backport and a demonstration recipe.