WhatsOnMyBookshelf is an online book trading application that makes it possible for users to exchange books using a simple point system. For avid readers who have rapidly growing collections of books they've already read, this is a clever little app to help them get their hands on new reads without having to spend any actual cash. WhatsOnMyBookshelf also provides the WhatsOnMyBookshelf API that provides access to this functionality.
Flexpaper is an open source document viewer--and it is beautiful. The Flexpaper API lets you incorporate your publications into other applications. Flexpaper works on virtually any device and is, no surprise here, flexible. It can display in HTML, HTML5, Adobe Flash or any combination of these.