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:

Allows you to display the CiviCRM menubar on non-civicrm pages of your website. This module creates an invisible Drupal Block that loads the CiviCRM menubar. By default the block will display the menubar on every non-civicrm page of your website. Simply edit the block to refine exactly which pages the menubar appears on and which users have permission to see it....
Current Usage: 78
From 22 April 2013, UK based charities and Community Amateur Sports Clubs can sign up to make repayment claims electronically. The new service, Charities Online, is being introduced in response to customer feedback. It will make repayment claims faster and easier by filing online. The current R68(i) print and post repayment form will be replaced by three options for making claims....
Current Usage: 77
Adds bounce handling, and mailing reports for CiviCRM's transactional emails and creates Activities for mail such as Receipts and Invoices.
Current Usage: 74
This extension has been incorporated into core as of CiviCRM 4.6, so the 4.5 version will not be updated....
Current Usage: 72
“Simple Donate” extension which allows the administrator to link existing donation pages with our new sleek, user friendly and responsive donation page....
Current Usage: 72
  THIS EXTENSION HAS BEEN REPLACED: This Drupal-only extension has been superseded by an improved version which is CMS-independent (CiviCRM-native): CiviCRM activity iCalendar feed.  Please consider using that extension instead....
Current Usage: 71
Introduction The Capitalisation Corrector is a CiviCRM extension that creates a scheduled job that corrects the capitalisation of contact's first and last name. Only contacts of the type 'Individual' are corrected and only names that are in all lowercase or all uppercase will be corrected....
Current Usage: 70
This extension changes the default so it doesn't send a confirmation email when registering a contact to an event from the back office. Nothing more,  really simple but avoids sending emails by mistake....
Current Usage: 70
This extension allows members of the public to apply for Grants. Administrators can create and administer Grant Application Pages in a manner similar to Contribution Pages. Custom Profiles for Grants and Contacts, including core and custom fields defined for grants and contacts, are added to Grant Application Pages....
Current Usage: 64
Automatically corrects email addresses when they're added, and allows for mass correction. Rules can be configured.
Current Usage: 60
Sagepay Payment Processor...
Current Usage: 57
eWay processor supporting recurring and one-off payments. This processor is recommended over the one that ships with core....
Current Usage: 55
This module provides an easy way to change the Drupal user to CiviCRM contact associations through the Drupal user page. Drupal project page....
Current Usage: 47
This extension gives you postcode look up when adding or editing contacts and configuring event location. Supported provideres are AFD, CiviPostcode, experian and postcodeAnywhere...
Current Usage: 46
Extension to facilitate postcode lookup feature in CiviCRM using AFD, Civipostcode, Experian or PostcodeAnywhere providers....
Current Usage: 46