These are a couple of online CRM systems which you can play with for free. While they do contact management as well as any other system, often the areas where these free systems fall behind is in the ability to configure and in automated workflow.
While SugarCRM is open source, I do not consider it free in that there are still costs of installation, hardware and setup. Online CRM’s have some of these costs but they are a fraction of an in-house solution’s.
This relatively unknown CRM system allows for unlimited users. Its quite rigid in its layout and function but covers the basics.
Zoho have online apps for practically every office need. Zoho CRM is free for the first three users and covers the essentials, including advanced features such as workflow. Think of Zoho as Google Apps and Salesforce combined and at no cost.