CiviCRM

CiviCRM is an open source and freely downloadable constituent relationship management solution. CiviCRM is web-based, open source, internationalized, and designed specifically to meet the needs of advocacy, non-profit and non-governmental groups. Integration with both Drupal and Joomla! content management systems gives you the tools to connect, communicate and activate your supporters and constituents.

Homepage: http://civicrm.org