A start-up called Shopkick Inc., for example, has signed up Best Buy Co. and Macy's Inc. as launch partners for a new kind of app for iPhone and Android handsets that detects when shoppers are in or near stores and offers rewards targeted to them. Shopkick exploits the phones' location-sensing abilities—and cameras that customers can use to scan bar codes on items—to offer product information, coupons or other marketing offers when shoppers are in a convenient position to buy.