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:

Create flexible forms on your Drupal site that integrate seamlessly with CiviCRM contacts, events, activities, cases, contributions, grants & memberships.
Current Usage: 3,311
This extension will allow Admins to create popup windows with any content, to any user, on any particular page.
Current Usage: 3,158
This extension integrates Mosaico an open source, responsive email template builder, with CiviCRM.
Current Usage: 2,451
CiviCRM Shoreditch Theme
Current Usage: 2,287
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: 1,985
Extended report framework including priceset reports.
Current Usage: 1,825
Allows the admin to create and manage discount codes that can be used on membership and event pages.
Current Usage: 1,737
Exposes CiviCRM data as true Drupal entity types, providing seamless integration with any module using Drupal's Entity API, such as Rules.
Current Usage: 1,578
Stripe Payment Processor - Accept payments by Credit Card
Current Usage: 1,223
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: 1,164
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: 1,157
Prevents users from overwriting their own record while trying to add a new participant / contribution / membership.
Current Usage: 1,048
Provides the latest version of the API (Application Programming Interface) for CiviCRM.
Current Usage: 1,030
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: 916
Extends custom data by creating a tab of fields that total up and summarize donation history for each contact.
Current Usage: 905