One of my favourite websites which I used to go on a few years ago was a website called "SocialVibe which was basically a digital advertising company which donated money to a charity chosen by the user when you complete various activities. For example the activities ranged from completing surveys to sharing advertisements to your friends.
It's basically an advertising technique which encourages users to get involved as the whole concept of giving to charity is somewhat cathartic. As a result the site got thousands of users and there was almost new brand endorsements every week as the site became more popular.
For the advertiser it became a platform in which to gather primary and quantitative research about their audience, so the whole process is a somewhat win,win process for both the advertiser and audience.
I really like this approach to advertising and in my opinion it makes your audience less passive than they would be with traditional advertising methods.
This concept has started making me think about other ways in which I could somewhat replicate this process with other digital interactive artifacts.