Skulpt is Python implemented in JavaScript: http://www.skulpt.org/

Once this is done, I'd like to make a website that allows a user to request an arbitrary number of nested interpreters.

That's really nice. But why is it so hard to write a "console" in Javascript. You know, like a python interpreter. Enter python, press enter, next line prints the result.

Yeah, I know it's hard. I tried it myself, and it's a pain in the neck. Just asking that's all.