There's a dark horse in the race to customer innovation through application programming interfaces (APIs). Through APIs, banks, healthcare firms, telcos, retailers, logistics firms, and many others are joining the likes of Facebook, Twitter, and Google in creating an API economy. Although industry discussion centers heavily on open web APIs, others like B2B APIs and internal APIs are just as important. But there's little industry discussion about a fourth and quite valuable category: product APIs. This report illustrates the difference between product APIs and the other three categories and highlights the unique angle they provide for finding and capturing customer value.