Innovation in web service APIs is not always technical, sometimes it's about money. Such is the case with Yahoo's announcement that later this year they will begin charging for usage of BOSS (Build your Own Search Service), their search web service for developers. Given that search revenue is typically ad-driven, and that advertising doesn't typically mesh with APIs, this has meant that search APIs have been one of the most challenging classes of APIs for providers to monetize. So it's notable to see how Yahoo has chosen to tackle this: via a 'utility' pricing model, somewhat akin to how infrastructure services like Amazon Web Services charge by usage.
Our API directory now includes 96 stocks APIs. The newest is the Eurex VALUES API. The most popular, in terms of directory page views, is the Bloomberg API. In terms of the technical details, REST leads the way. Below are more stats from the directory, including the entire list of stocks APIs.