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:

This powerful and popular module provides a smart form builder and processor with many features that are missing from CiviCRM's profile forms. Its many uses range from creating a simple newsletter sign-up form to managing a complex multi-step online application workflow....
Current Usage: 2,913
This extension will allow Admins to create popup windows with any content, to any user, on any particular page.
Current Usage: 1,214
Exposes many CiviCRM entities as true Drupal entities, any module that uses Drupal entities can manipulate CiviCRM data, Drupal style.
Current Usage: 1,188
Allows Drupal's cron to invoke CiviCRM's cron.
Current Usage: 530
Displays CiviCRM events in a calendar using the "fullcalendar" jQuery plugin.
Current Usage: 483
  THIS EXTENSION HAS BEEN REPLACED: This Drupal-only extension has been superseded by an improved version which is CMS-independent (CiviCRM-native): CiviCRM activity iCalendar feed.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: 133
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: 121
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: 118
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: 107
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: 73
Sunlight Congressional Districts - Fetches data about the current members of Congress using the Sunlight Congress API.http://services.sunlightlabs.com/docs/Sunlight_Congress_API/...
Current Usage: 70
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: 47
This module allows for taxonomy vocabularies and terms in Drupal to be mirrored with groups in CiviCRM....
Current Usage: 43
  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: 40
Drupal project page: http://drupal.org/project/civicrm_private_report...
Current Usage: 34