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
This extension integrates Mosaico an open source, responsive email template builder, with CiviCRM.
Current Usage: 2,765
FlexMailer is an email delivery system for CiviCRM that is used by Mosaico and some other extensions.
Current Usage: 2,666
CiviCRM Shoreditch Theme
Current Usage: 2,470
Extended report framework including priceset reports.
Current Usage: 2,068
Allows the admin to create and manage discount codes that can be used on membership and event pages.
Current Usage: 1,736
CiviRules is an extension that allows rules based actions, like automatically adding a contact to a group when he/she contributes more than
Current Usage: 1,567
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
This extension is a utility for allowing angular pages to load the backbone profile editor/selector widget. It comes prebuilt with a service for loading backbone and necessary files as well as a directive to turn a standard input into the profile widget.   ...
Current Usage: 1,447
Stripe Payment Processor - Accept payments by Credit Card
Current Usage: 1,388
Extends custom data by creating a tab of fields that total up and summarize donation history for each contact.
Current Usage: 962
This iATS Payments extension as of 1.7.x provides payment processors for iATS Payments 1stPay as well as for iATS Payments Legacy Services.
Current Usage: 915
Exports reports and search results directly into the native MS Excel format (xlsx).
Current Usage: 870
Prevents users from overwriting their own record while trying to add a new participant / contribution / membership.
Current Usage: 862
Extension to support General Data Protection Regulation
Current Usage: 860