The first version of the Datamuse API is now generally available and free for everyone to use! See http://www.datamuse.com/api/ for more info, including complete docs and examples.
If you're a developer interested in adding some kind of word search feature to your app, you might find the API useful. It's already been used in two assistive writing tools, a few search boxes (for intelligent autocomplete), and word games. It also powers key parts of RhymeZone and OneLook.
Over the past 20 years we've received hundreds of requests for some kind of programmatic access to the data behind our sites from developers eager to make their own writing-oriented apps. This API finally gives you access to this data, and much more. We're excited to see what else people create with this service, and we look forward to adding more apps of our own in the near future.