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 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: 482
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: 455
Provides integration with Clickatell SMS gateway for sending single and mass messages.
Current Usage: 430
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: 369
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: 324
This extensions makes the permission flag on a contact's relationship work as a true ACL.
Current Usage: 312
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: 312
CiviCRM GiftAid Extension
Current Usage: 309
Provides WordPress widgets for a single event or a list of events, in the sidebar or with shortcodes.
Current Usage: 300
Have you ever wanted to rearrange the contact summary screen? Move the most important information to the top? Remove unnecessary stuff? Create a simplified layout for your volunteers and interns but a more robust layout for your fundraising team? This extension will let you do just that.
Current Usage: 295
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: 292
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: 282
Provides email delivery for email from CiviCRM via MailChimp's Mandrill service.
Current Usage: 282
This extension provides additional mail merge tokens for CiviCRM that pull in CMS content....
Current Usage: 268
This Extension uses the jquery plugin Slicknav to make the CiviCRM administration menu web responsive. Currently there is one breakpoint at 50em at which point the menu changes from a bar across the top to a hamburger style dropdown menu so that CiviCRM users can more easily navigate thru CiviCRM on small screens (tablets, phones etc.).  ...
Current Usage: 253