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:

The Advanced Fundraising extension provides a series of reports to help fundraisers view their progress in greater detail....
Current Usage: 290
Provides email delivery for email from CiviCRM via MailChimp's Mandrill service.
Current Usage: 282
Event Calendar Extension allows you to view all CiviCRM events in a calendar by month,day,week. The setting page allows us to select which events should be shown on calendar with the color we want for particular event type. Also the title for Event Calendar. Below is the Screenshot of Setting Page of Event Calendar ...
Current Usage: 277
Normalise the data entered in several fields. So far firstname, last name... so they are Properly Capitalised What does it do? It modifies profiles and the normal contact edit/add so when you type (or paste) XAV as the first name, it converts it into Xav...
Current Usage: 276
This extension provides additional mail merge tokens for CiviCRM that pull in CMS content....
Current Usage: 274
Helper extension to manage settings for multiple entities This extension does not do anything on it's own. Rather it supports other extensions by storing settings related to entities without the requirement to create a table per extension....
Current Usage: 254
Upcoming: CiviCRM Gift Aid Extension v3.0 for 4.5+ CiviCRM Gift Aid Extension 3.0 is being developed by Compucorp Ltd....
Current Usage: 223
You like how tags are displayed in the contact summary? this extension does the same, for groups. You can also add the contact to a group from the summary (click on the Groups label)...
Current Usage: 215
This extension is designed to preserve the email reputation of your server and its IP by ensuring that all outbound email is sent from an address with the same domain as the System-generated Mail Settings From Email Address configured at Administer > Communications > Organization Address and Contact Info (civicrm/admin/domain?action=update&reset=1)....
Current Usage: 201
  THIS EXTENSION HAS BEEN REPLACED: This Drupal-only extension has been superseded by an improved version which is CMS-independent (CiviCRM-native): CiviCRM activity iCalendar feed.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: 133
Core searches on telephones and emails search only for the Primary telephone and Primary email. This extension provides a custom search that looks for primary and non-primary telephone and emails that match the criteria, and displays all of the phone numbers and emails for those contacts....
Current Usage: 133
CiviCRM Multiday Event creates multiple entity based events associated to a single Drupal node linked to a single CiviCRM Event. The duration of the CiviCRM Event last from the start date and time of the first session to the ending date and time of the last session. So if you have a class that meets every Friday in May, Drupal will display 4 events (5/4, 5/11, 5/18, and 5/25)....
Current Usage: 121
The Drupal core contact module provides a no-frills way for users to email you via a simple webform. If you like that module, but would also like those form submissions to be recorded in CiviCRM, then this no-frills extension is for you....
Current Usage: 118
The Database Health Report  provides you with a snapshot of your CiviCRM database's usage. It reports the last login time and number of contacts and activities modified by each user. WIth this report, you can evaluate the effectiveness of staff trainings, or get a quick glimpse into which users are active and who may need more prodding....
Current Usage: 116
This extensions makes the permission flag on a contact's relationship work as a true ACL.
Current Usage: 113