MochiKit includes an interactive interpreter demo that gives you an experience similar to what you get with Python's interactive interpreter.
MochiKit.Base makes comparison and representation in JavaScript more flexible and useful.
Base also provides a bunch of the functions that you know and love from Python such as map and filter.
JavaScript 1.7 includes Python-style iterators, but it will be a while before we have universal browser support for that version of JavaScript. MochiKit.Iter gives you that capability now.
MochiKit.Logging provides an easy way to debug your JavaScript that works in any browser.
Creating and working with Document Object Model (DOM) nodes is simplified by MochiKit.DOM.
MochiKit.Color knows how to convert between color representations so that you can get just the color you need to appear on the page.
JavaScript includes a Date object, but doesn't help you convert from common external formats. MochiKit.DateTime includes functions for converting dates and times with the outside world.