RhymeBrain displays the best rhyming and most commonly used words for a search query. The API allows users to include RhymeBrain.com rhyming results on their web site. It uses RESTful calls and responses are formatted in JSON.
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.