Virtual Currency affiliates
Social games like FarmVille or Moshi Monsters are played online by millions of users worldwide. Many players will integrate the game with their Facebook, Twitter or other social media profiles, creating competition and letting their connections see how well they're progressing. Often these games will utilise currency which allows the user to purchase goods to enhance the experience.
Virtual currency affiliates serve ads within these games, enticing the user to click through and make a purchase from the merchant of choice. These ads utilise affiliate links, and when a conversion takes place, the commission is paid back to the customer as virtual currency to be spent within the game. The banner might display a call to action such as "Buy from Merchant X today and earn 100 gold coins!" As ever with the affiliate channel, the attraction for the client is the cost-per-acquisition model - they'll only pay when the banners converts to a sale.