To make it easier, we’ve grouped these into categories with the most popular first.

Although there are some similarities in features between these, depending on your situation, you might find one is more suitable to your needs.

Utilities for Web Developers

MooTools
MooTools is a modular JavaScript framework that makes writing powerful, flexible, and cross-browser code easy and fun.
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jQuery
jQuery: The Write Less, Do More, JavaScript Library
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Bootstrap
Powerful, extensible, and feature-packed frontend toolkit. Build and customize with Sass, utilize prebuilt grid system and components, and bring projects to life with powerful JavaScript plugins.
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Knockout.js
KnockoutJS is a JavaScript library that helps you create rich, responsive display and editor user interfaces with a clean underlying data model.
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Utilities for Developers

Prototype
Prototype is a JavaScript framework that makes it easy to create rich, dynamic web applications with less code.
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Cocoa Controls
Cocoa Controls is a library of components and controls for iOS and Mac OS developers. It's the largest collection of open source controls, helping developers save time and focus on building their apps.
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