I built the micro-payment web startup called Cred using Ruby on Rails, and PHP for the WordPress plugin. It features a documented REST API for integration.
Cred allows any content producer/publisher to upload their content into a content management system (like WordPress), and then paywall that content by installing the Cred plugin. For the readers, they simply sign into YourCred.com, then browse any affiliated site, and click to redeem cred from their account in one click. It’s a very simple, highly cost-effective system for managing micro-payments.
This was the most complete web startup I’d ever built, which included video demos, demo sites, and fully working transactional system. Sadly, I had to can this project in 2012 due to lack of resources.