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.
Our API directory now includes 56 billing APIs. The newest is the Merchee API. The most popular, in terms of mashups, is the Stripe API. We list 5 Stripe mashups. Below you'll find some more stats from the directory, including the entire list of billing APIs.