The C# (.NET) library for Windows is for use with the Bloomberg API. The Bloomberg API provides free, unrestricted access to raw financial data and is part of their Open Market Data Initiative. The library is offered under a "free-use" license, which allows unrestricted access to the interface's header file, documentation, and programming examples. Extensive documentation, including code examples, is available in the Developer Documentation and at the API website.
There are so many jobs listed for developers familiar with the Paypal API on GetAFreelancer that the site has a category dedicated to it. And that's not all--there are over 300 API-related listings on the site, which connects freelancers with people who need programmers and designers.