Oxford Landing wine out of South Australia makes it easier for customers to become repeat buyers of their products. Their labels include a tear-off card to remember the wine by. I rememer in the days of the Rolodex (now, there is a company that missed the boat...they had the brand that could have lead them to create Act! or Plaxo, but that is another post), marketing literature would often include a die-cut Rolodex card that people could tear off and save. This is the wine bottle equivalent. Brilliant!

I think product packaging is one of the most underutilized mediums for viral marketing. I'd love to hear about other examples that you have seen.