The USAePay Android Library provides an interface for running server to server processing. The Library is for developing websites using the client side method, which provides a stable UI. This library is written in Java.
Payleven, the Square equivalent for the world outside of the US, has launched an API to complement its mobile payment device. Prior to the API, use of Payleven's mobile payment device required downloading the Payleven app (similar to Square's current model). However, the API allows developers to integrate Payleven's functionality within third party apps.
2Checkout, a worldwide leader in payment services, has released a new Payment API. The API allows online merchants to integrate 2Checkout’s payment processing engine directly into third party websites. Because the API is directly integrated with third party websites, merchants can maintain a customized user experience that best suits business needs.