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:

Multisite (url) based permissioning.   This is a port of the drupal multisite module to an extension. It allows the CiviCRM multisite concept to be extended such that the site a contact logs onto will affect which contacts they can see and edit....
Current Usage: 213
This extension adds a pivot report that suppots multiple CiviCRM entities.
Current Usage: 204
Displays CiviCRM events in a calendar using the "fullcalendar" jQuery plugin.
Current Usage: 163
Core searches on telephones and emails search only for the Primary telephone and Primary email. This extension provides a custom search that looks for primary and non-primary telephone and emails that match the criteria, and displays all of the phone numbers and emails for those contacts....
Current Usage: 133
The CiviCalendar extension is an CiviCRM component which allows visualizing different types of cases, events and activities.
Current Usage: 125
The Database Health Report  provides you with a snapshot of your CiviCRM database's usage. It reports the last login time and number of contacts and activities modified by each user. WIth this report, you can evaluate the effectiveness of staff trainings, or get a quick glimpse into which users are active and who may need more prodding....
Current Usage: 116
Automatically corrects email addresses when they're added, and allows for mass correction. Rules can be configured.
Current Usage: 103
What is CiviBooking? Are you a non profit with Rooms or Resources that you think you could raise money by renting out? Or are you a community center or voluntary service looking to manage your resources better? Sick of spreadsheets, emails and that Google calendar that no one keeps up to date? Then CiviBooking is the extension for you!...
Current Usage: 95
This extension improves the default search in several ways: more efficient requests search in the start of the names instead of in the middle (first or last) adjust based on the length of the query and the number of results to do the query(ies) that are the most efficient Give it a go and let me know what you think of the result....
Current Usage: 92
A tool that allows you to easily correct invalid phone numbers.
Current Usage: 89
The Drupal core contact module provides a no-frills way for users to email you via a simple webform. If you like that module, but would also like those form submissions to be recorded in CiviCRM, then this no-frills extension is for you....
Current Usage: 86
This is a civicrm extension to export directly into an excel format instead of the default csv. If you never had any problem with opening a csv into excel, this extension won't help you (and you are likely to be a US user dealing with english names)...
Current Usage: 85
From 22 April 2013, UK based charities and Community Amateur Sports Clubs can sign up to make repayment claims electronically. The new service, Charities Online, is being introduced in response to customer feedback. It will make repayment claims faster and easier by filing online. The current R68(i) print and post repayment form will be replaced by three options for making claims....
Current Usage: 77
Implements hook_username_alter() so that, if available and non-empty, the CiviCRM "Display name" is shown in place of the Drupal login name for username displays....
Current Usage: 77
This extension has been incorporated into core as of CiviCRM 4.6, so the 4.5 version will not be updated....
Current Usage: 72