BBDO Interactive Montreal came up with this great viral for La Fédération des Producteurs de Lait du Québec (Quebec Milk Producers Federation) : create the longest chain of chocolate milk drinkers. You simply send in a short clip of you, grabbing on a cold chocky with your right hand and chugging it, then passing it on with your left-- thus creating the chain impression. People are invited to vote for the best clips, whom will deserve Video iPODs and other prices.
Very simple, functional idea: puts the consumer as a star, easy to participate in, and definitely entertaining. Check out "infiniment bon" (french only).