In today's "World of Mouth" and "Social Marketing" power giving products away for a social favour could promote your product better than thousands of dollars promotion budgets. Twitter Payment is Free to use, you can create unlimited Pay with a Tweet buttons.
Use Twitter & Facebook
You can choose to require either a Twitter status update on the users Twitter profile or a Post to the user's Facebook Wall in order to make them able to download your products, books, etc. By letting users do the promotion for you, you will receive a lot more visitors to your website.