Free ISBN Web Service
From The Programmable Web comes word of ISBNdb.com, a large database of books indexed by ISBN. Of interest to developers is the fact that there is a complete (and free) REST API. Items in the site are organized (and accessible) via book, subject, category, author, and publisher index.
Several (ok, many) ECS developers have asked me for similar services over the past year or two and it is great to see this new and powerful service in action. If you build something cool with this (alone or in conjunction with ECS) please drop me a note so that I can give it consideration for this blog.