Sprintf.js
sprintf.js is a complete open source JavaScript sprintf implementation.
jQuery Number
Easily format numbers for display use. Replace numbers inline in a document, or return a formatted number for other uses.
Augment.js
Modern JavaScript in your IE
Object.observe
Object.observe polyfill/shim trying to be compliant
Sugar
A Javascript library for working with native objects.
Placeholder Shiv
Tiny polyfill for the placeholder form element attribute. Requires Prototype.js.
Taggd
A tool to show and add tags to images
Big.js
A small, fast JavaScript library for arbitrary-precision decimal arithmetic.
Core Lib
A port of the Ruby 1.9.3 corelib to coffeescript/javascript/node that conforms to rubyspec.org.
Klass
a utility for creating expressive classes in JavaScript
Hitch
A lightweight engine that allows you to hitch new capabilities to the browser (prollyfill).
Css3 Finalize
Tired of adding prefix for each broswer when it comes to the new css3 attributes?
Args.js
Create javascript functions with optional, default, grouped and named parameters.
CSS Parent Selector
CSS4 parent selector based on jQuery
Placeholders.js
A JavaScript polyfill for the HTML5 placeholder attribute
Console.message
Console messages for cool kids
Es6 Shim
ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines