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: 3,305
This extension integrates Mosaico an open source, responsive email template builder, with CiviCRM.
Current Usage: 2,253
CiviCRM Shoreditch Theme
Current Usage: 2,017
Extended report framework including priceset reports.
Current Usage: 1,628
Allows the admin to create and manage discount codes that can be used on membership and event pages.
Current Usage: 1,609
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: 1,605
Exposes CiviCRM data as true Drupal entity types, providing seamless integration with any module using Drupal's Entity API, such as Rules.
Current Usage: 1,388
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: 1,062
Stripe Payment Processor - Accept payments by Credit Card without worrying about PCI compliance
Current Usage: 1,061
Prevents users from overwriting their own record while trying to add a new participant / contribution / membership.
Current Usage: 1,014
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: 954
Extends custom data by creating a tab of fields that total up and summarize donation history for each contact.
Current Usage: 852
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: 849
This is a payment processor extension for the iATS Payments payment processing service, making use of the web services interface.
Current Usage: 816
This extension adds the possibility to export directly into the MS Excel (.xlsx) format, instead of CSV (less fiddling, easier to use). It supports exporting from reports and from search results....
Current Usage: 785