jQuery!

Zepto is a minimalist JavaScript library for modern browsers with a largely jQuery-compatible API.

Durandal is a cross-device, cross-platform client framework written in JavaScript and designed to make Single Page Applications (SPAs) easy to create and maintain.

Intuitive and DocumentedIt is strongly influenced by jquery with many similar methods so you'll feel at ease developing with Umbrella.However there are also some improvements such as with .ajax(), .append(), etc.

Underscore is a utility-belt library for JavaScript that provides a lot of the functional programming support that you would expect in Prototype.js (or Ruby), but without extending any of the built-in JavaScript objects. It's the tie to go along with jQuery's tux, and Backbone.js's suspenders.

Lo-Dash is a drop-in replacement for Underscore.js that delivers performance improvements, bug fixes, and additional features.

Dojo Toolkit is an open source modular JavaScript library (or more specifically JavaScript toolkit) designed to ease the rapid development of cross-platform, JavaScript/Ajax-based applications and web sites.

MooTools (My Object-Oriented Tools) is a lightweight, object-oriented, web-application framework for JavaScript.

DHTMLX is a JavaScript library that provides a comprehensive set of UI widgets and makes building web apps easy. Develop killer web apps with DHTMLX!

The Harp Platform is the easiest way for anyone to create an exceptional website. Landing Pages, galleries, blogs, e-commerce, domains, hosting, analytics, 24/7 support - all included.

Free web application framework for building Web sites and Web applications using JavaScript, HTML and CSS.

Chaplin is an architecture for JavaScript applications using the Backbone.js library. Chaplin addresses Backbone’s limitations by providing a lightweight and flexible structure that features well-proven design patterns and best practices.

Ractive.js is different. It solves some of the biggest headaches in web development – data binding, efficient DOM updates, event handling – and does so with almost no learning curve.