Ask HN: What's your favorite unofficial API?

One of my favorite hackathon projects was using an unofficial Robinhood API (https://github.com/sanko/Robinhood) to ask Alexa how a current stock was doing. If it had increased over the last year, I would ask Alexa to "purchase X shares of X stock".

Writing joke apps is a fun way to try out a new programming language and laugh about it.