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:

Powerful and very popular module provides a smart form builder and processor leveraging Drupal Webform module for CiviCRM.
Current Usage: 3,304
Exposes CiviCRM data as true Drupal entity types, providing seamless integration with any module using Drupal's Entity API, such as Rules.
Current Usage: 1,522
Allows Drupal's cron to invoke CiviCRM's cron.
Current Usage: 503
This is a CiviCRM extension designed for large scale donation receipt generation in a heavily regulated environment (e.g Germany).
Current Usage: 118
Allows you to display the CiviCRM menubar on non-civicrm pages of your website.
Current Usage: 74
Aegir module to automatically setup Drupal instances with CiviCRM.
Current Usage: 62
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: 61
A simple user interface to help with creating personalised links when sending emails from CiviCRM.
Current Usage: 58
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: 44
Online Worldpay Payment Processor using API to generate tokens and authorise payment. Once installed just set-up the payment processor and this will be available for using in CiviCRM...
Current Usage: 28
This CiviCRM extension automatically creates a Smart Group of Registered Participants to an Event when a New Event is created. Example Use Case...
Current Usage: 14
This module allows for content types in Drupal to be mirrored with groups in CiviCRM. Dependencies This module requires the CiviCRM module....
Current Usage: 14
This extension will allow Admins to create popup windows with any content, to any user, on any particular page.
Current Usage: 12
This module allows for users to access event payment receipts based on event contribution records in CiviCRM. Receipts are created as Drupal nodes so site admins can enable printing/PDF exports.
Current Usage: 9
CiviCRM and Drupal both place a copy of jQuery on the page, which is inefficient. This module removes Drupal's (older) copy and uses the single copy from CiviCRM for both applications. This results in faster page loads and a more responsive web browser.
Current Usage: 5