CiviCRM is a web-based, open source, Constituent Relationship Management (CRM) software geared toward meeting the needs of non-profit and other civic-sector organizations. It is a highly customizable platform that integrates seamlessly with Drupal, and can be used to manage donors, members, events, mass emailing and much more.
We like to use CiviCRM for membership organizations and organizations managing a high volume of paid or free events that require online registration.