I've never heard of them before, but was looking to buy an app and this was offered as a payment option.
Essentially, a service that takes care of the "free ipod" type of program. So you can offer your customers to either buy your product for $29.95 or free with a signup to one of the Trial Pay offers (USA Today, Wall Street Journal, etc).
http://www.trialpay.com/online-services/offer-based-payments/