Script.aculo.us - A collection of Web 2.0 style JavaScript libraries that help web developers to easily add visual and ajax effects to projects. Requires Prototype. Prototype - Provides class-style Object Oriented and AJAX, freely distributable under the terms of an MIT-style license. Mootools - Offers a lightweight web2.0 JavaScript framework. Dojo - A Toolkit by the Dojo Foundation featuring DHTML and AJAX functions. The site features demos, downloads, help on getting started, documentation, support and community fundtions. SproutCore - Provides tools to create desktop like applications for the web. Sizzle - A pure-JavaScript CSS selector engine designed to be easily dropped in a host library. D3.js - Data-Driven Documents - A JavaScript library for manipulating documents based on data using HTML, SVG and CSS. Site includes overview, examples, documentation, and source download. JavaScriptMVC - Built on top of jQuery, has a collection of practices and tools for building JavaScript applications. Sencha - Creates development frameworks and tools that help design, develop and deploy applications for desktop and mobile devices. YUI - Set of utilities and controls for building richly interactive web applications. MochiKit - Provides a suite of libraries taken from Python. Documentation and download. Ember.js - A framework for creating ambitious web applications. Knockout - Simplify dynamic JavaScript UIs with the Model-View-View Model (MVVM) pattern. Agility.js - Maintainable and reusable browser code without the verbose or infrastructural overhead found in other MVC libraries. Spine - Lightweight MVC library for building JavaScript applications. petermichaux/maria - MVC framework for JavaScript applications. Sammy.js - A Small web framework. Stapes.js - MVC microframework. Epitome - Extensible and modular open-source MVC* framework, built out of MooTools Classes and Events. soma.js - Scalable javascript framework. Plastronjs - MVC framework for Closure Library and Closure Compiler. rAppid.js - Declarative rich Internet application JavaScript MVC framework. Serenade.js - A light and compact client-side framework for building web pages. Cappuccino Web Framework - Build Desktop Class Applications in Objective-J and JavaScript. qooxdoo - Universal JavaScript framework for websites, mobile, desktop and the server. paulca/eyeballs.js - A slim javascript library designed to sit on top of a javascript framework, such as jQuery or Prototype. PureMVC - JavaScript MultiCore Framework - For creating applications based upon the classic Model-View-Controller design meta-pattern. batman.js - Built with Rails development in mind. RequireJS - Module loader. Zepto.js - Minimalist JavaScript library for modern browsers with a largely jQuery-compatible API. Tower.js - Small components for building apps. jHERE - Simple and powerful map API in the form of a jQuery (or Zepto.JS, or Tire) plugin.