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:

Customize CiviCRM's QuickSearch
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CallHub is a hosted phone banking service that enables your staff to talk with your contacts over the phone banking campaign and store notes about the call....
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This is a CiviCRM extension designed for "heavy duty" donation receipt generation. Find the user documentation at http://wiki.civicrm.org/confluence/display/CRM/DonationReceipts+Extension Features:...
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SparkPost / CiviCRM Integration Integrates SparkPost to CiviCRM, so email can be sent out over the SparkPost service and bounces can be processed in CiviCRM. * Now processes bounces for Transactional and Bulk (CiviMail) emails....
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