Dwolla is an extremely innovative Payment API and the site allows you to transfer money with one of the least payment charges. We have covered Dwolla before here. Here is an interview with Michael Schonfeld, Developer Evangelist of Dwolla API.
Visa has opened up its global payments network to developers releasing a set of new RESTful APIs and developer website. Developers will have access to different Visa platform capabilities via RESTful APIs including payments, analytics, alerts, ATM locations, geographical-based data, and more.