Stringformatter
JS string formatter that supports objects, currency, date/time, decimals, and more supports easy extension and garbage collection ... goes far beyond sprintf approach.
Babel
Babel is a compiler for writing next generation JavaScript.
Svgeezy
Allows for automatic Image fallbacks for browsers that can't handle SVGs
Hitch
A lightweight engine that allows you to hitch new capabilities to the browser (prollyfill).
Placeholders.js
A JavaScript polyfill for the HTML5 placeholder attribute
Poly
Small, fast, awesome. The only ES5-ish set of polyfills (shims) you can mix-and-match because they're individual AMD modules.
Store
A better way to use localStorage and sessionStorage
Es6 Shim
ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines
CSS Parent Selector
CSS4 parent selector based on jQuery
Selectivizr
Selectivizr is a JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8.
Respond
A fast & lightweight polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more)
Placeholder Shiv
Tiny polyfill for the placeholder form element attribute. Requires Prototype.js.
HTML5shiv
This script is the defacto way to enable use of HTML5 sectioning elements in legacy Internet Explorer.
Yepnope.js
An Asynchronous Conditional Resource Loader
Flexie
Legacy support for the CSS3 Flexible Box Model
ES5 Shims
ECMAScript 5 compatibility shims for legacy JavaScript engines
CSS3 Pie
A behavior for Internet Explorer allowing it to recognize and render various CSS3 box decoration properties