RhymeBrain is a service made to help writers, or anyone else for that matter, find rhymes for words, make new portmanteaus, and alliterate well. When writing with figurative language, or writing poems, such a tool can be very helpful for a writer who's currently stuck. It might also be helpful for such things as computer generated literature, but for that, you'd want a robust API with full access to the database. Well, you're in luck, because it does indeed exist.
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.
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.