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
Allows the admin to create and manage discount codes that can be used on membership and event pages.
Current Usage: 1,486
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
Extended report framework including priceset reports.
Current Usage: 1,138
This extension integrates Mosaico an open source, responsive email template builder, with CiviCRM.
Current Usage: 1,032
Prevents users from overwriting their own record while trying to add a new participant / contribution / membership.
Current Usage: 796
CiviCRM Shoreditch Theme
Current Usage: 783
Stripe payment processor for CiviCRM.
Current Usage: 759
Provides tools for signing up, managing and tracking volunteers
Current Usage: 728
Civisualize is a civiCRM extension which let you visualize your civiCRM data with the power of dc.js and d3.js. dc.js let users create graphs which enable filtering through the data with just a click on the graphs giving you a lot of powers....
Current Usage: 725
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: 695
Extends custom data by creating a tab of fields that total up and summarize donation history for each contact.
Current Usage: 635
Provides a way to keep Mailchimp Lists and CiviCRM groups in sync.
Current Usage: 616
FlexMailer is an email delivery system for CiviCRM v4.7+. It does not define any user interface; rather, it replaces some internal parts of CiviMail and provides new APIs. Other extensions can use these APIs to provide more fine-grained control over email delivery. A few example scenarios supported by FlexMailer:...
Current Usage: 614