Over at www.concordfreepress.com a free-conomy model with a twist. Here is what they have to say about it:

Our first book—Give and Take by novelist Stona Fitch—is out and circulating through the world. We're distributing 1,500 copies free via online requests and our network of independent bookstores. In return, we ask readers to make a voluntary donation to a charity or someone in need. And we ask them to pass the book on, so that every time the book changes hands, it generates more contributions.