I designed and built experimental multiplayer hypertext user-extendable environment. Imagine a hybrid between an Twine and an old-school text MUD.
The front end is jQuery; the back end is Python and MongoDB; they’re connected by a websocket. Players can build new content using a wiki-style text editor and Python as the game scripting language.
To make this work, I had to implement a safe subset of Python in Python. This was surprisingly easy. On the down side, since this is a 2013 project, it’s Python 3.4. The prototype works great but it would need a lot of reworking to scale beyond a few dozen users.