For a limited time, to build up a following, they are offering this program on any app in the App Store. The site is legitimate and I have already received my rebate, which was sent directly to my PayPal account.
How does this work? Essentially the app developers cover the cost of the rebate (meaning they lose the 30% commission taken by Apple), and in return their apps are pushed up the charts. By climbing the charts, the app is more exposed and more people are likely to buy it, thus covering the loss taken during the rebate period.
I'm definitely going to be a fan of this site, and can't wait to see what apps they give away starting next month!