It is interesting to track new APIs and mashups at Programmable Web. I think it is one of the most useful resources on the web. Using these APIs you can build your own little applications called mashups.
Today I came across one that shows an interesting trend developing. It is a programming interface for accessing books, subjects, publishers and authors. The site lists some impressive statistics:
Associated with the API are a few xml vocabularies.
An XML vocabulary is useful for exchanging data in a common format. When data is stored in XML format, it is more amenable to access and manipulation. As these formats spread in popularity, there is a likelyhood that more people will start using these formats. These are the baby steps in the making of a data web.