Drupal finally began replacing NIH with PIE. Compare Drupal's 8.x branch a year ago with it today. A year ago, we had one class of 3rd party PHP code, in an uninteresting corner of the update system. Today, it contains 3rd party code from no less than five different projects: Symfony, Doctrine, Guzzle, Assetic, and Twig. Those libraries are included via Composer, the new and ground-breaking PHP-library-management-system-that-actually-finally-works. Code from at least one more should be landing soon.
Drupal developers, working on Drupal, have contributed to a number of other projects, including Symfony and Symfony CMF