IIRC Doug Orleans once made an ifMUD bot for a version of Zendo where a rule was a regular expression. This would give the user a way to express their guess of the rule instead of you having to test them on examples (regex equality is decidable).
Also I made a version over s-expressions and Lisp predicates -- it was single-player and never released. It would time-out long evaluations and treat them as failure. I wonder if I can dig up the code...
LW has often discussed the inductive logic game Zendo, as a possible way of training rationality. But I couldn't find any computer implementations of Zendo online.
So I built two (fairly similar) games inspired by Zendo; they generate rules and play as sensei. The code is on GitHub, along with some more explanation. To run the games you'll need to install Python 3, and Scikit-Learn for the second game; see the readme.
All bugfixes and improvements are welcome. For instance, more rule classes or features would improve the game and be pretty easy to code. Also, if anyone has a website and wants to host this playable online (with CGI, say), that would be awesome.