Announcement - Valve
Later this week, new menu options will appear within Steam as we initiate the closed beta for Guest Passes, a new feature that will allow owners of certain Steam-enabled games (purchased either via Steam or at retail) to send a friend limited-time trials for those games.
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.