The Xero Ruby Library xeroizer provides developers a way to communicate their ruby/rails based applications with the Xero API. There's information on development for both public and private applications.
Bitwage launched an API that aims to ease delay times and high fees traditionally associated with paying wages across international borders. API access to its payroll platform enables companies to integrate Bitwage solutions into existing systems such as time tracking software and other workflows.
Nations Trust Bank, one of the largest banks in Sri Lanka, recently announced an open banking initiative through its Open API Banking Platform. The bank hopes that the platform will help change the closed minded approach to banking that has historically dominated the island nation's banking.
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.