Hi, I’ve written a gamebook engine in JavaScript (link). The reader’s experience in the story is fairly similar to Twine’s output, but it’s much more open-ended in terms of what can be added or revised in the current scene (at least, in my experience with Twine). To write stories in it, you only need HTML knowledge (though JavaScript/jQuery functions are allowed as well).
It features smooth text transitions, an optional single (non-URL-based) save game per story, conditional passages, and all sorts of possible actions when the reader clicks a link or when a particular passage is loaded.
Feedback is welcome. I can also host stories on my site if anyone wishes.