Composer is proven dependency management tool in PHP. Just mention application's all specifications and dependencies in composer.json and run "composer install". Composer locates, validates, downloads and loads the specified packages and ensures that exactly the right versions for each package is used.
Drupal 8 has adapted various 3rd party PHP libraries which are used AS-IS for implementation and most importantly are a part its CORE. Assetic for asset management, Composer for dependency management, Doctrine for Annotation and ORM, EasyRDF for Resource Descriotion Format markups, Guzzle for REST service call and HTTP Client, PHPUnit for unit testing and PSR-3 for Universal logging.