|Why They're Hot:
50% of transactions at Groupon occur on mobile devices, and mobile customers spend 50% more than web-only customers. Groupon celebrated its fifth birthday in November 2013 by redesigning its sites and apps. The beautifully designed and efficient apps are key to Groupon's success. 50 million people worldwide have downloaded Groupon apps, the third-most in retail, Arbitron says. Even Groupon's m-commerce site looks like an app. The iPad app has a clean, easy two-column layout. Touch a product image and up pops a window with HD images, specs galore, a button to go social, and more. Groupon runs 10 tests concurrently on features in each app version; results guide the next version's design, says Fabio Sisinni, director of product management, mobile.