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.