Best way to integrate Python and JavaScript?

Is it possible to integrate Python and JavaScript? For example, imagine you wanted to be able to define classes in JavaScript and use them from Python (or vice versa). If so, what's the best way? I'm interested not only if this is possible but if anyone has done it within a "serious" project or product.

I'm guessing it would be possible using Jython and Rhino, for one example, but I'm curious whether or not anyone's ever actually done this, and if there are solutions for other platforms (especially CPython).

How about pyjs?

From the above website:

pyjs is a Rich Internet Application (RIA) Development Platform for both Web and Desktop. With pyjs you can write your JavaScript-powered web applications entirely in Python.

Here's something, a Python wrapper around the SeaMonkey Javascript interpreter... http://pypi.python.org/pypi/python-spidermonkey