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:

You like how tags are displayed in the contact summary? this extension does the same, for relationships.    ...
Current Usage: 183
This CiviCRM extension provides a daily scheduled job named "Call Email2Each.Sendemails API (Daily)"  This job expects 2 parameters: a group name and a message template name.  When this job runs,  it will send an email to each contact in the group/smart group. The contents of the email will be the message template. ...
Current Usage: 172
Adds over 20 new fields to Summary Fields, including contributions by related contacts, plus fields for CiviMail, CiviEvent, and CiviGrant.
Current Usage: 168
Allows adding an automatically calculated "Additional Percentage" field to any price set (e.g., add 3% to cover credit card fees).
Current Usage: 154
A tool that allows you to easily correct invalid phone numbers.
Current Usage: 153
PDF API for CiviCRM to create a PDF file and send it to a specified e-mail address. This is usefull for automatic generation of letters The entity for the PDF API is Pdf and the action is Create. Parameters for the api are specified below:...
Current Usage: 152
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: 151
Onscreen tutorials for learning to use CiviCRM
Current Usage: 144
This extension makes it easy to have all email sent from Outlook stored on the appropriate contacts' records as Email Activities in CiviCRM. Code in Outlook talks directly to CiviCRM native APIs. The next version of the integration (hopefully out early fall 2015) will integrate Outlook Calendar with CiviCRM....
Current Usage: 143
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: 143
The CiviCase v5 extension is an overhaul of the CiviCase UI. It provides a richer experience with more thoughtful layouts and interactions.
Current Usage: 140
Automatically corrects email addresses when they're added, and allows for mass correction. Rules can be configured.
Current Usage: 138
Adds bounce handling, and mailing reports for CiviCRM's transactional emails and creates Activities for mail such as Receipts and Invoices.
Current Usage: 137
This extension tries to make public bank data like name, BIC code, and national bank ID accessible for CiviCRM. The information itself will be downloaded from the various national sources, and can be updated any time. The information will be stored in a new option group....
Current Usage: 135
Store documents on the contact dashboard A document can be linked to more than one contact Version management with a document Custom search to find documents  ...
Current Usage: 135