For their latest marketing scheme, Valve is using the tried and true 'Free Trial' method. Valve has announced their new Steam Guest Pass system, which allows owners of certain Valve games to send out free trial passes to their friends. Day of Defeat: Source should be the first game available once the open beta starts.
"During the beta period the new menu items will appear on all Steam accounts, though the ability to send the Guest Pass will be inactive. When this closed beta concludes, Valve will extend the Steam Guest Pass Beta to Day of Defeat: Source owners.
Similar to the Free Weekends, Guest Passes are another example of advanced service and functionality that can only be offered through Steam, the leading platform for content delivery and management."