From their site: Prosper, America's first people-to-people lending marketplace, was created to make consumer lending more financially and socially rewarding for everyone. The way Prosper works is intuitive to people who have used eBay. Instead of listing and bidding on items, people list and bid on loans using Prosper's online auction platform.
Prosper, America's first peer-to-peer lending marketplace, launched a new set of API services that enable lenders to develop tools that enhance the ability to reach investment goals. The API provides over 550 data points that lenders can use to evaluate investment opportunities. Propser expects users to integrate the Prosper API into existing tools or build new tools based on the API and its underlying data set.