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:

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: 271
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: 267
This CiviCRM extension provides a daily scheduled job named "Call Email2Each.Sendemails API (Daily)"  This job expects 2 parameters: a group name and a message template name.  When this job runs,  it will send an email to each contact in the group/smart group. The contents of the email will be the message template. ...
Current Usage: 254
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: 253
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: 253
The CiviMobileAPI extension (com.agiliway.civimobileapi) is a CiviCRM component that allows to use a mobile application CiviMobile.
Current Usage: 249
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: 238
The Advanced Fundraising extension provides a series of reports to help fundraisers view their progress in greater detail....
Current Usage: 238
Extension to facilitate postcode lookup feature in CiviCRM using AFD, Civipostcode, Experian, PostcodeAnywhere or getaddress.io providers.
Current Usage: 232
Sagepay using the Omnipay multiprocessor extension.
Current Usage: 231
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: 231
Omnipay is an open source payment processor library that supports multiple payment processors (around 30 currently). https://github.com/omnipay/omnipay...
Current Usage: 231
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: 220
Creates Tax Receipts with distribution by email or print, detailed reporting, press ready
Current Usage: 217
This extension allows a staff user to add, edit and delete line items of a contribution.
Current Usage: 214