Revel Systems API makes it possible to run a cash register, a restaurant and a mobile point-of-sale system with an iPad as the center of the user interface. Check out what Revel has to say about the API on their website.
The LemonStand API provides the ability to integrate its ecommerce tools into almost any website, and is aimed at the midsize merchant market. Built with Php, MySQL and Ajax, it is completely customizable using HTML and CSS. Updates are achieved through a single click. Lemonstand promises an upcoming App store for third party wares.