Sometimes it takes a little human intervention to make semantic applications easier to build. The Guardian newspaper has augmented its Open Platform API with unique identifiers for bands and books. In turn, the company has simplified the process of creating a mashup that uses multiple sources to focus on a single work, which may help its content spread farther.
It's always exciting to see those outside the web industry using an API to improve their sites. NY Times First Look blog has a great example of what Dallas Public Library has done with the Best Sellers API.
A team at Indiana University created Scholarometer to "facilitate citation analysis and help evaluate the impact of an author's publications." The crowdsourced data created through the Scholarometer platform is available through the Scholarometer API that allows developers to track certain authors through name, citation, disciplines, relevance, and other metrics.