An implementations of Langton's Ant written in Javascript (in the browser).
Check the project Wiki for (slightly) more details.
To run this, simply clone the repo, and load up ant-board.html in your browser.
Development is a little more complicated; you should be able to just load up SpecRunner.html to run the tests, but this may not work, depending on what browser you're using (it definitely doesn't work under Chrome).
The problem is the test coverage tool I'm using (blanket). It pulls in the project code (to instrument it) via XHR requests, and Chrome gets all pissy about allowing XHR requests when the original page was loaded from a file. Other web browsers may be just fine; this is a rather grey area.
There are various workarounds for this problem; the one I'm using is to simply run a small web server.
The script to start that up is in scripts/server. It fires up a copy of Ruby, loads the Webrick server (packaged with Ruby, so you shouldn't have to install anything else), and listens on port 8000.
So, after running scripts/server, point your web browser at http://localhost:8000/SpecRunner.html to run the tests.
n.b. Do not run this on an Internet-facing machine!
Webrick, by itself, is not a production-grade web server. If you run it so it's visible on the public Internet, you are effectively hanging out a "Hack my machine, please." sign.