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:

Click & Pledge Payment Processor for CiviCRM.
Current Usage: 4
Provides WordPress widgets for a single event or a list of events, in the sidebar or with shortcodes.
Current Usage: 0
CiviCRM contribution pages allow you to generate a "widget" showing the progress toward a goal. This plugin makes it easy to include one or more contribution page "widgets" as actual WordPress widgets on your sidebar. This plugin requires CiviCRM 4.3 or higher....
Current Usage: 0
CiviCRM WordPress Member Sync keeps a WordPress user in sync with a CiviCRM membership by granting either a role or capabilities to a WordPress user who has that membership. This enables you to have, among other things, members-only content on your website that is only accessible to current members as defined by the membership types and status rules that you set up in CiviCRM....
Current Usage: 0
A port of the Drupal 'civicrm_og_sync' module for WordPress that enables two-way synchronisation between BuddyPress Groups and CiviCRM....
Current Usage: 0
CiviCRM Admin Utilities modifies CiviCRM's behaviour and appearance in single site and multisite installs....
Current Usage: 0
The CiviCRM WordPress Profile Sync plugin keeps the "First Name", "Last Name", "Email Address" and "Website" fields of a WordPress (and BuddyPress) user profile in sync with the corresponding fields of a CiviCRM contact. The synchronisation takes place regardless of whether the changes are made in WordPress, BuddyPress or CiviCRM....
Current Usage: 0