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 facilitates uploading CSVs via the CiviCRM import interface. Useful for bulk uploading events or campaigns etc. The civicrm api entity can be set to, for example, Event or Campaign. (Note that for entities like Contribution Pages you need to use API names - either ContributionPage or contribution_page). The same permissions apply as for any api create call....
Current Usage: 469
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: 466
Provides integration with Clickatell SMS gateway for sending single and mass messages.
Current Usage: 437
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: 384
This CiviCRM extension normalizes your data before it is saved to the database. It therefore works from the back-end, with public form submissions such as profiles, contribution, event registration and membership creation/renewal pages as well as with data imports. It can normalize first, middle, last and company names, phone numbers, and addresses including zip codes....
Current Usage: 383
This extension allows users to get their CiviCRM assigned activities into their Google Calendar or similar calendaring applications, by providing a feed of each user's assigned activities in iCalendar format, ready to be subscribed to from within Google Calendar, Outlook, etc....
Current Usage: 376
This extension adds a block to the contact summary that allows editing of any configured relationship type with a UI similar to the employer field.
Current Usage: 344
This extensions makes the permission flag on a contact's relationship work as a true ACL.
Current Usage: 315
CiviCRM GiftAid Extension
Current Usage: 310
Provides WordPress widgets for a single event or a list of events, in the sidebar or with shortcodes.
Current Usage: 300
This extension provides additional mail merge tokens for CiviCRM that pull in CMS content....
Current Usage: 299
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: 291
Provides email delivery for email from CiviCRM via MailChimp's Mandrill service.
Current Usage: 282
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: 281
This Hebrew Calendar extension for CiviCRM has a variety of functionality,  including mail merge tokens for sending yahrzeit reminders, birthday wishes, Hebrew demographic information, Hebrew date of birth, next Hebrew birthday, yahrzeits, APIs and more....
Current Usage: 273