Extensions are installable packages which give CiviCRM new functionality, and this directory provides a centralized list of extensions which the CiviCRM community has created.

Further resources:

Create flexible forms on your Drupal site that integrate seamlessly with CiviCRM contacts, events, activities, cases, contributions, grants & memberships.
Current Usage: 3,357
Exposes CiviCRM data as true Drupal entity types, providing seamless integration with any module using Drupal's Entity API, such as Rules.
Current Usage: 1,396
Allows Drupal's cron to invoke CiviCRM's cron.
Current Usage: 570
Displays CiviCRM events in a calendar using the "fullcalendar" jQuery plugin.
Current Usage: 370
Implements hook_username_alter() so that, if available and non-empty, the CiviCRM "Display name" is shown in place of the Drupal login name for username displays....
Current Usage: 94
The Drupal core contact module provides a no-frills way for users to email you via a simple webform. If you like that module, but would also like those form submissions to be recorded in CiviCRM, then this no-frills extension is for you....
Current Usage: 86
CiviCRM Multiday Event creates multiple entity based events associated to a single Drupal node linked to a single CiviCRM Event. The duration of the CiviCRM Event last from the start date and time of the first session to the ending date and time of the last session. So if you have a class that meets every Friday in May, Drupal will display 4 events (5/4, 5/11, 5/18, and 5/25)....
Current Usage: 79
Drush module to automatically setup Drupal instances with CiviCRM using Aegir. https://www.drupal.org/project/hosting_civicrm...
Current Usage: 61
  THIS EXTENSION HAS BEEN REPLACED: This Drupal-only extension has been superseded by an improved version which is CMS-independent (CiviCRM-native): CiviCRM activity iCalendar feed.  Please consider using that extension instead....
Current Usage: 59
Allows you to display the CiviCRM menubar on non-civicrm pages of your website. This module creates an invisible Drupal Block that loads the CiviCRM menubar. By default the block will display the menubar on every non-civicrm page of your website. Simply edit the block to refine exactly which pages the menubar appears on and which users have permission to see it....
Current Usage: 58
This module provides an easy way to change the Drupal user to CiviCRM contact associations through the Drupal user page. Drupal project page....
Current Usage: 53
  When a customer buys something from a Drupal Commerce online shop, this module: Creates a CiviCRM contact record if the purchaser doesn't already exist. Adds the purchaser to the specified CiviCRM group. Adds (or updates) a contribution history to their record. ...
Current Usage: 42
CiviRelate creates a CiviCRM relationship between the acting user and a CiviCRM contact created by the acting user through the submission of a designated CiviCRM profile form. CiviRelate has a simple configuration page to define the Profile to use and the relationship to create....
Current Usage: 33
This module allows for taxonomy vocabularies and terms in Drupal to be mirrored with groups in CiviCRM....
Current Usage: 23
This module allows for content types in Drupal to be mirrored with groups in CiviCRM. Dependencies This module requires the CiviCRM module....
Current Usage: 22