Drupal core examples project (https://drupal.org/project/examples)
.. and a searchable and filterable list of contributed modules. What’s neat about this? You can filter by “example” modules for implementations outside of core: https://www.drupal.org/project/project_module?f%5B0%5D=&f%5B1%5D=&f%5B2%5D=im_vid_3%3A19440&f%5B3%5D=&f%5B4%5D=sm_field_project_type%3A%5B*+TO+*%5D&text=&solrsort=iss_project_release_usage+desc&op=Search