Gakusei

Gakusei is an online (and real-time) tool to study and test your Japanese knowledge. It can generate thousands of different sentences by rearranging words and sentences, although its vocabulary is a bit limited right now.

Try it online

On your browser: learn.fragments.me

There's also an android app wrapper for your pleasure.

Or run it locally

$ meteor

(if you don't have meteor, head up here https://www.meteor.com/)