Knockout makes it easier to create rich, responsive UIs with JavaScript
Give your JS App some Backbone with Models, Views, Collections, and Events
Ember.js - A JavaScript framework for creating ambitious web applications
Lightweight MVC library for building JavaScript applications
YUI 3.x Source Tree
Javascript MVC for the "write less, do more" programmer
The official read-only mirror of Dojo Core.
Claypool is a small, fast, railable Javascript Application Framework , built on jQuery that provides all the usual important patterns for large, long-lived client-side apps, server-side apps, or something strangely, beautifully in the middle.