Do you remember the billionth app downloaded on the App Store? It was Bump, a free app that lets you swap pictures and contact info by bumping two iPhones together. The concept was simple, the app was cheap, and lo and behold: people like gimmicks. For a while, Bump was a chart-topper. PayPal has long had a free app on the App Store, which provided mundane features like managing your PayPal [...]