Many marketers run a contest, award the prize and move on, but Volvo has done a good job of maintaining audience involvement in the treasure hunt promotion they created in conjunction with the Pirates of the Caribbean movie. There's new material when you return to the web site, and plenty of explanation for newcomers who missed the boat but still want to know what happened. Right now they are posting videos taken during the final stages of the contest. They used many good storytelling techniques to keep non-players entertained.
Yahoo! Finance: Volvo Premieres Webisodes Capturing Worldwide Treasure Hunt for Buried Volvo, 2006-Aug-24, Volvo Press Release
The webisodes were produced in the tradition of such popular reality shows as Survivor and Amazing Race, and were filmed by individuals who have worked on reality shows such as Fear Factor..., and will be available online for the coming months.