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 is a simple CiviCRM report giving you a list of the upcoming birthdays of your contacts. Simply install the extension, create a new report, and use "Available for Dashboard" to give you the list upon login....
Current Usage: 264
CiviSEPA allows you free SEPA direct debit collections, if you are in Europe.
Current Usage: 256
This extension allows a staff user to add, edit and delete line items of a contribution.
Current Usage: 253
When viewing reports, clicking on a contact's name brings you to the contact's record, not their Contact Detail Report or Contribution Detail Report....
Current Usage: 248
This extension adds a pivot report that suppots multiple CiviCRM entities.
Current Usage: 239
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: 238
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: 229
This extension loads United States counties when you first install it.  Once enabled, it does nothing.  You may disable it or uninstall it, and nothing will happen: the counties will remain loaded in your database.  If you re-enable it, it will check for missing counties and just add those....
Current Usage: 225
This extension permits a contact to be changed from one type to another.
Current Usage: 223
Creates Tax Receipts with distribution by email or print, detailed reporting, press ready
Current Usage: 215
The Advanced Fundraising extension provides a series of reports to help fundraisers view their progress in greater detail....
Current Usage: 207
Places recently-viewed items in the Menu bar for quick access.
Current Usage: 200
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: 199
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: 197
Extension to facilitate postcode lookup feature in CiviCRM using AFD, Civipostcode, Experian, PostcodeAnywhere or getaddress.io providers.
Current Usage: 186