I've recently had the opportunity to install and work in Vtiger, an open source source CRM system. VTiger is a split of the more well-known SugarCRM. SugarCRM has a community version of its software that is open source, but it doesn't include all the feature that you find in the commercial version. This seems to be a major sticking point for those seeking a full featured open source CRM.
Out of the box, vtiger provides much of what you would get in a "out of the box" salesforce.com installation. Some features include: