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 module allows for content types in Drupal to be mirrored with groups in CiviCRM. Dependencies This module requires the CiviCRM module....
Current Usage: 23
SMS API for CiviCRM to send sms through the API The entity for the SMS API is SMS and the action is Send. Parameters for the api are specified below:...
Current Usage: 23
  Contact Calendar is a CiviCRM extension that displays activities where the contact is an assignee or a target contact. The calendar is useful when the activities are in the future and people can plan thier schedule.  ...
Current Usage: 23
PDF API for CiviCRM to create a PDF file and send it to a specified e-mail address. This is usefull for automatic generation of letters The entity for the PDF API is Pdf and the action is Create. Parameters for the api are specified below:...
Current Usage: 21
Adds the ability to setup recurring payments for contacts through the administration interface....
Current Usage: 20
Great custom search for searching and filtering on event participants in CiviCRM. Easily list just the participants who choose "session AAA"  from your priceset options and send them all an email or SMS/Text message.  Or get the total count of participants for priceset option "session BBB" so you know how many chairs and snacks you need to set up.  Current Features: ...
Current Usage: 20
Thanks to funding from The Royal Television Society, this extension allows maximum number of attendees / additional participants to be configured for an event....
Current Usage: 19
Integrate CiviCRM Events with GoToWebinar. https://github.com/veda-consulting/uk.co.vedaconsulting.gotowebinar/blob......
Current Usage: 19
This extension supports CiviXero but has been written so that it could also support different accounting synchronisations in the hope it will encourage greater collaboration. It is a dependency of CiviXero It provides...
Current Usage: 19
This extension provides batch update functionality for grants similar that for contacts in CiviCRM core. Up to 100 grants can be edited simultaneously in a table-like grid on a webpage. The fields are defined by adding them to a profile. NB. This extension adds support for contact reference fields as custom grant fields in profiles, allowing them to be used for batch updates....
Current Usage: 19
This extension provides integration with Xero accounting package. It requires the supporting extension nz.co.fuzion.accountsync - more details are in the README file General functionality...
Current Usage: 19
This extension syncs all CiviCRM contacts to the Contacts Directory in Google Apps. The Contacts Directory is shared between all users of your Google Apps domain, and hence all CiviCRM contacts are available to them in the Google Mail autocomplete, or for search on their mobile devices....
Current Usage: 19
This extension adds logging to the civirules environment. It does by hooking into the logging mechanism of Civirules. Requirements: CiviCRM CiviRules ...
Current Usage: 18
Optimized Autocomplete is a CiviCRM extension. If you have a large database (over 500,000 contacts), you may have noticed that the autocomplete contact search box in top left of CiviCRM will never finish. This is because by default the MySQL queries that are being called to do the search are inefficient on large databases....
Current Usage: 18
Support for this extension is on hold
Current Usage: 17