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
Extends the CiviCRM membership component so that when a membership is created or renewed a record for the membership “period” is recorded.
Current Usage: 1,576
The aim of this extension is to extend the CiviCase component functionality to update the case status any time.
Current Usage: 1,530
Exposes many CiviCRM entities as true Drupal entities, so that almost any module that can use Drupal entities can also access and manipulate CiviCRM data, Drupal style.
Current Usage: 970
Allows the admin to create and manage discount codes that can be used on membership and event pages.
Current Usage: 931
Provides tools for signing up, managing and tracking volunteers
Current Usage: 579
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
Extended report framework including priceset reports.
Current Usage: 468
Prevents users from overwriting their own record while trying to add a new participant / contribution / membership.
Current Usage: 449
Provides a way to keep Mailchimp Lists and CiviCRM groups in sync.
Current Usage: 362
Stripe payment processor for CiviCRM.
Current Usage: 344
Provides integration with Clickatell SMS gateway for sending single and mass messages.
Current Usage: 315
Provides WordPress widgets for a single event or a list of events, in the sidebar or with shortcodes.
Current Usage: 300
Extends custom data by creating a tab of fields that total up and summarize donation history for each contact.
Current Usage: 297