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:

Extension to facilitate postcode lookup feature in CiviCRM using AFD, Civipostcode, Experian or PostcodeAnywhere providers....
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
  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: 43
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
Improved reports on mailing statistics
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
This is a CiviCRM extension to make it possible for a Nagios monitoring system to track the functioning of a CiviCRM site.  Initially, services tracked are:...
Current Usage: 39
Over-riding tpls is awful & hard to sustain. But if you HAVE to do it this extension could help. By default CiviCRM allows you to override the tpl for a given contribution page by putting the file in the path yourcustomtemplatefolder/CRM/Contribute/Form/Contribution/   If you want to override ONLY contribution page 6 then...
Current Usage: 38
This extension suppresses name and address fields from the billing block if they are in a profile and uses the profile fields instead. It prefers address fields that are more likely to be billing fields....
Current Usage: 37
Extension deals with situation where sites have many short-term contribution pages that may later be disabled. Sometimes these pages are google indexed and people go to them wanting to donate....
Current Usage: 37
More info to come - but this extension provides custom searches written for the Australian Green Party by Andrew McNaughton to help manage Australian contact data by finding spam users and addresses in need of some manual checking....
Current Usage: 36
SMS API for CiviCRM to send sms through the API
Current Usage: 35
CiviHR is affordable HR software for non-profit organisations.
Current Usage: 35
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
This module makes it easier to configure settings in CiviCRM by:...
Current Usage: 32