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.
Everybody likes money, but nobody likes the inconvenience of having to visit different sites to manage all their accounts. Redwood City-based Yodlee has been aggregating financial data for its MoneyCenter users and partner companies since 1999, and is now expanding its developer services to include "Accelerator" and "Incubator" programs, both of which augment its existing API offerings.
Mastercard recently announced the Mastercard Innovation Engine. The engine acts as a marketplace for next generation digital customer experiences. The marketplace allows Mastercard and collaborators to offer API access to customer-focused digital experiences through plug and play solutions.