MochiKit!

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!

Ext JS is a JavaScript library for building interactive web applications using techniques such as Ajax, DHTML and DOM scripting. Ext includes interoperability with jQuery and Prototype.

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

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.

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

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.

Enyo is a JavaScript app framework enabling developers build native-quality HTML5 apps that run everywhere — from phones and tablets to PCs and TVs.

qooxdoo is a universal JavaScript framework that enables you to create applications for a wide range of platforms. With its object-oriented programming model you build rich, interactive applications (RIAs), native-like apps for mobile devices, light-weight traditional web applications or even applications to run outside the browser.

No lock-in, no wrapper objectsDesigned to work with Vanilla JS, not replace it. Light as a featherOnly 3KB minified & gzipped!Chaining without prototype pollutionOnly adds one _ property (optional, customizable) ExtensibleAdding new blissful methods is easy as pie!(Mostly) readable sourceKnow what you’re running. Excellent for learning! Built with modern standardsUse polyfills to extend browser support.

AJS is a ultra lightweight JavaScript library that is around 2 years old. It is inspired by MochiKit, but differs by being small and expressive. The current version is only 33 KB (uncompressed!), whereas MochiKit and other libraries such as JQuery, Mootools or Prototype is over/around 100 KB.

JayData is a standards-based, cross-platform JavaScript library and a set of practices to access and manipulate data from various online and offline sources.

Nombo is an Open Source Node.js framework that allows you to easily build Rich Cloud Applications.

PKIjs is a pure JavaScript library implementing the formats that are used in PKI applications (signing, encryption, certificate requests, OCSP and TSP requests/responses). It is built on WebCrypto (Web Cryptography API) and requires no plug-ins.