Who's using the Datamuse API?

RhymeZone RhymeZone, the leading rhyming dictionary, uses the API extensively: for search autocomplete; for finding similar-sounding words when a search term is not recognized; for spelling correction; for finding descriptive words; and for its "synonyms / related" search results tab.
OneLook OneLook, a popular meta-dictionary tool, was the original motivation for the Datamuse API. The reverse dictionary / thesaurus section hits nearly every feature of the API. OneLook also uses it for search autocomplete and for the "related words" features that appear on its search results pages.
Voice Dream Writer Voice Dream uses the Datamuse API to find words by sound and by meaning for users of its critically acclaimed assistive writing app.
SmashWords SmashWords, the world's largest distributor of indie ebooks, uses the Datamuse API for search autocomplete.
RapPad RapPad uses the API for contextual word suggestions in its innovative writing tool.
Syllacrostic solver This project by a student at Rensselaer Polytechnic Institute uses the Datamuse API for relevancy scoring.
WikSearch This is an experimental Wikipedia search interface that uses the Datamuse API on a 10 million term vocabulary of English-language Wikipedia titles.
Third-party libraries A few people have written client libraries for the Datamuse API: There's now Datamuse4J for Java; python-datamuse for Python; a module for node.js; and a module for Drupal.