If there is something that makes great CMS like Drupal in these days is, undoubtedly, the number of modules or plugins availables, whether made by the community or by professionals who try to live with it. 

Focusing on Drupal extensions, the quantity of modules currently available is huge, more than 18000. Therefore, it's good to have a guide to those who are essential to almost any web project.

Mandatory list of modules for any Drupal website