To launch the new VW Polo in South Africa they created a game of tag using twitter and a bunch of the New Polo cars.
For 6 days people could spot the new Polo around a specified route, tag it on twitter, and drive it, right there and then. The person who drove it for the longest, without being tagged, won the new Polo. Work by Ogilvy Cape Town.
p.s I may be a little biased in likening this as I work there.