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:

Event Calendar Extension allows you to view all CiviCRM events in a calendar by month,day,week. The setting page allows us to select which events should be shown on calendar with the color we want for particular event type. Also the title for Event Calendar. Below is the Screenshot of Setting Page of Event Calendar ...
Current Usage: 472
This extension facilitates uploading CSVs via the CiviCRM import interface. Useful for bulk uploading events or campaigns etc. The civicrm api entity can be set to, for example, Event or Campaign. (Note that for entities like Contribution Pages you need to use API names - either ContributionPage or contribution_page). The same permissions apply as for any api create call....
Current Usage: 460
Provides integration with Clickatell SMS gateway for sending single and mass messages.
Current Usage: 434
This extension allows users to get their CiviCRM assigned activities into their Google Calendar or similar calendaring applications, by providing a feed of each user's assigned activities in iCalendar format, ready to be subscribed to from within Google Calendar, Outlook, etc....
Current Usage: 391
This CiviCRM extension normalizes your data before it is saved to the database. It therefore works from the back-end, with public form submissions such as profiles, contribution, event registration and membership creation/renewal pages as well as with data imports. It can normalize first, middle, last and company names, phone numbers, and addresses including zip codes....
Current Usage: 387
This extension adds a block to the contact summary that allows editing of any configured relationship type with a UI similar to the employer field.
Current Usage: 367
Normalise the data entered in several fields. So far firstname, last name... so they are Properly Capitalised What does it do? It modifies profiles and the normal contact edit/add so when you type (or paste) XAV as the first name, it converts it into Xav...
Current Usage: 350
CiviCRM GiftAid Extension
Current Usage: 340
This extensions makes the permission flag on a contact's relationship work as a true ACL.
Current Usage: 318
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: 298
This Hebrew Calendar extension for CiviCRM has a variety of functionality,  including mail merge tokens for sending yahrzeit reminders, birthday wishes, Hebrew demographic information, Hebrew date of birth, next Hebrew birthday, yahrzeits, APIs and more....
Current Usage: 293
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: 290
This extension provides additional mail merge tokens for CiviCRM that pull in CMS content....
Current Usage: 289
You like how tags are displayed in the contact summary? this extension does the same, for groups. You can also add the contact to a group from the summary (click on the Groups label)...
Current Usage: 281
Helper extension to manage settings for multiple entities This extension does not do anything on it's own. Rather it supports other extensions by storing settings related to entities without the requirement to create a table per extension....
Current Usage: 283