Slashdot links to a BusinessWeek article that introduces open source and reports on companies using open source to save money.  Some popular software they list as cost-conscious include Linux (operating system), Apache (server), MySQL (relational database), Firefox (web browser), Xen (virtualization), Pentaho (business intelligence), (full office suite), Drupal (content management system), Alfresco (content management system), SugarCRM (customer relationship management), and Asterisk (telephone switch/PBX).