Michael Koploy of Software Advice has posted a useful list of the 9 best iOS apps for retailers. He notes the App Store rating and the number of ratings an app has received, giving you idea which are the most popular. The free app Square makes his list and continues to be one of the best deals I've seen. You not only get the app free, but also a free credit card reader that connects to your phone. And there's no monthly fee or contract. You simply pay per transaction.