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 module makes it easier to configure settings in CiviCRM by  ...
Current Usage: 32
This extension makes it easy to have all email sent from Outlook stored on the appropriate contacts' records as Email Activities in CiviCRM. Code in Outlook talks directly to CiviCRM native APIs. The next version of the integration (hopefully out early fall 2015) will integrate Outlook Calendar with CiviCRM....
Current Usage: 32
Integrate Direct Debit payments using SmartDebit (https://www.smartdebit.co.uk/) with CiviCRM
Current Usage: 31
This CiviCRM extension provides two way synchronisation between CiviCRM groups and MailChimp Lists. Changes to users in regular groups are synced with MailChimp in real-time, smart groups use CiviCRM's Scheduled Jobs to sync up. Changes made on MailChimp are sync'd back to CiviCRM using Webhooks...
Current Usage: 29
SYSTOPIA OpenStreetMap CiviCRM Geocoder This CiviCRM extension adds the Nominatim OpenStreetMap server as a geocoding option. Just install the extension, and activate via "Administer" => "System Settings" => "Mapping and Geocoding"....
Current Usage: 28
This CiviCRM extension allows you to limit the display of individual prices in price sets to specific groups. This allows you to implement things like members-only pricing for events. (by using a Smart Group to track current members) This extension works with both static and smart groups....
Current Usage: 27
Small helper module for migrating a CiviCRM database for v 4.2 or lower....
Current Usage: 27
Komfortables Erstellen von Einzel- und Sammelbescheinigungen für steuerlich absetzbare Zuwendungen. Weitere Informationen zu Einsatzgebiet, Funktionsweise, Bedienung und Entwicklungsstand finden sich auf unserer offiziellen Webpage der Spendenbescheinigungs-Extension....
Current Usage: 27
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: 26
This module allows for taxonomy vocabularies and terms in Drupal to be mirrored with groups in CiviCRM....
Current Usage: 24
CiviRelate creates a CiviCRM relationship between the acting user and a CiviCRM contact created by the acting user through the submission of a designated CiviCRM profile form. CiviRelate has a simple configuration page to define the Profile to use and the relationship to create....
Current Usage: 24
  Contact Calendar is a CiviCRM extension that displays activities where the contact is an assignee or a target contact. The calendar is useful when the activities are in the future and people can plan thier schedule.  ...
Current Usage: 23
PDF API for CiviCRM to create a PDF file and send it to a specified e-mail address. This is usefull for automatic generation of letters The entity for the PDF API is Pdf and the action is Create. Parameters for the api are specified below:...
Current Usage: 21
Adds the ability to setup recurring payments for contacts through the administration interface....
Current Usage: 20
This module is used to "certify" participants of CiviEvents. A certification in this context is adding a CiviCRM contact to a CiviCRM group and keeping a record of that transaction. Certifications can occur automatically as participants change status (using CiviCRM API), or in bulk for any event by using the 'sync' tab on the Certification Rule....
Current Usage: 20