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 listing displays modules specifically built to integrate with features from Backdrop. You can also browse directories of plugins or modules that are made specifically for Drupal, Joomla and WordPress. The recommended way to add functionality to CiviCRM is using CiviCRM extension.

This module allows a user to create a field that will allow selection of a CiviCRM Contribution or Event Registration page, which can then be inserted onto the page when the field is displayed.
Integrate Ubercart customers/purchases with CiviCRM.
A simple way to use Backdrop's cron to call CiviCRM's cron.
This module will provide simple synchronization between Backdrop node content and contacts in CiviCRM Groups based on rules defined by the administrator.
This module allows a user to insert a core CiviCRM Contribution page into a Backdrop Layout via a block.