Online bookseller Erapublications has an API for its in2Era web service for selling ebooks through online distributors. According to its documentation, this RESTful API, which requires an authentication key, integrates its catalog onto your site. Distributors get a cut of the sales.
Developers were angered when Yahoo Finance deactivated a CSV download feature that was being treated as an API. As a result, it appears as though several applications broke and developers voiced their disappointment. But, as it turns out, they were taking a big risk in the first place.