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: 337
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: 333
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: 309
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: 289
Provides email delivery for email from CiviCRM via MailChimp's Mandrill service.
Current Usage: 282
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: 271
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: 268
This extensions makes the permission flag on a contact's relationship work as a true ACL.
Current Usage: 265
Displays CiviCRM events in a calendar using the "fullcalendar" jQuery plugin.
Current Usage: 263
CiviCRM GiftAid Extension
Current Usage: 252
This Extension uses the jquery plugin Slicknav to make the CiviCRM administration menu web responsive. Currently there is one breakpoint at 50em at which point the menu changes from a bar across the top to a hamburger style dropdown menu so that CiviCRM users can more easily navigate thru CiviCRM on small screens (tablets, phones etc.).  ...
Current Usage: 251
Provides the latest version of the API (Application Programming Interface) for CiviCRM.
Current Usage: 250
In version 4.7 and up, CiviCRM ships with only CKEditor, and other wysiwyg editors can be added via extension. This extension makes TinyMCE available as a wysiwyg editor for CiviCRM....
Current Usage: 228
If you are in Europe and use CiviCRM to manage recurring contributions, you need this extension. Warning: upgrading to 1.2.4: If you're using the payment processors, please verify and re-save your payment processor settings! If you've been using the SEPA tokens, you'll have to adjust your templates SEPA Direct Debit Module...
Current Usage: 215