TrueAccord, an upstart debt collection agency, is using an API to make debt collection a smoother, less painful process for both creditors and debtors.
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.
Handshake, sales order processing platform, offers the Handshake API that allows developers to integrate Handshake with third party applications and services. Handshake allows users to write, accept, and sync orders across any device or server running the Handshake platform. Additionally, Handshake supports barcode scanning for more streamlined order processing.