Babel
Babel is a compiler for writing next generation JavaScript.
Store
A better way to use localStorage and sessionStorage
Poly
Small, fast, awesome. The only ES5-ish set of polyfills (shims) you can mix-and-match because they're individual AMD modules.
Augment.js
Modern JavaScript in your IE
Yepnope.js
An Asynchronous Conditional Resource Loader
Placeholder Shiv
Tiny polyfill for the placeholder form element attribute. Requires Prototype.js.
CSS Parent Selector
CSS4 parent selector based on jQuery
Es6 Shim
ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines
Css3 Finalize
Tired of adding prefix for each broswer when it comes to the new css3 attributes?
Selectivizr
Selectivizr is a JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8.
Stringformatter
JS string formatter that supports objects, currency, date/time, decimals, and more supports easy extension and garbage collection ... goes far beyond sprintf approach.
HTML5shiv
This script is the defacto way to enable use of HTML5 sectioning elements in legacy Internet Explorer.
Respond
A fast & lightweight polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more)
Hitch
A lightweight engine that allows you to hitch new capabilities to the browser (prollyfill).
ES5 Shims
ECMAScript 5 compatibility shims for legacy JavaScript engines
Placeholders.js
A JavaScript polyfill for the HTML5 placeholder attribute
Flexie
Legacy support for the CSS3 Flexible Box Model