Amazon has cemented itself as a top tier option for charging users thanks to its Amazon Flexible Payments Service API. The crowd-funding site Kickstarter has run hundreds of millions through the API, so it should also get some credit. Although, at Amazon, Kickstarter may get blame instead, having inspired many other sites, which eventually caused Amazon to turn away sites with similar concepts.
PayPal has officially released PayPal X, its next generation platform that includes a variety of APIs and other resources for developers. Primarily aimed at allowing developers to embed payment processing in a wider array of applications and environments, this is a major expansion of PayPal's APIs.