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 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
Sunlight Congressional Districts - Fetches data about the current members of Congress using the Sunlight Congress API.http://services.sunlightlabs.com/docs/Sunlight_Congress_API/...
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
Extension to facilitate postcode lookup feature in CiviCRM using AFD, Civipostcode, Experian or PostcodeAnywhere providers....
Current Usage: 46
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
The helptab extension provides contextual help for CiviCRM: on any back-office CiviCRM page a little tab appears on the right side of your screen. Clicking on that tab will bring up a list of articles related to this page, clicking on the article's title will open it in a new tab....
Current Usage: 45
This module allows for taxonomy vocabularies and terms in Drupal to be mirrored with groups in CiviCRM....
Current Usage: 43
Improved reports on mailing statistics
Current Usage: 42
This extension tries to make public bank data like name, BIC code, and national bank ID accessible for CiviCRM. The information itself will be downloaded from the various national sources, and can be updated any time. The information will be stored in a new option group....
Current Usage: 42
This extension is to support the use of payment tokens with payment processors. It contains the part of token handling that I think core would ideally do & I will be adding to it. Currently it   1) creates a table civicrm_payment_tokens...
Current Usage: 40
  When a customer buys something from a Drupal Commerce online shop, this module: Creates a CiviCRM contact record if the purchaser doesn't already exist. Adds the purchaser to the specified CiviCRM group. Adds (or updates) a contribution history to their record. ...
Current Usage: 40