The PayChannel API makes it possible to incorporate its credit card processing gateway into your software platform. Public information on the API is not available, though it does provide 8 payment features.
The Web Payments Working Group has published the first public drafts of payment specifications which include a Payment Request API, payment method identifiers, and basic card payment. The basic idea behind the specifications is to make payments easier and more secure for end users.
How do you spot a mature platform? You look for its app store. Evernote, the platform for storing and accessing all kinds of things to remember, has created a gallery of products built with its API, which is expected to grow into a complete app store.