Babel
Babel is a compiler for writing next generation JavaScript.
Poly
Small, fast, awesome. The only ES5-ish set of polyfills (shims) you can mix-and-match because they're individual AMD modules.
Placeholders.js
A JavaScript polyfill for the HTML5 placeholder attribute
Augment.js
Modern JavaScript in your IE
Es6 Shim
ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines
Svgeezy
Allows for automatic Image fallbacks for browsers that can't handle SVGs
CSS Parent Selector
CSS4 parent selector based on jQuery
CSS3 Pie
A behavior for Internet Explorer allowing it to recognize and render various CSS3 box decoration properties
Selectivizr
Selectivizr is a JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8.
Placeholder Shiv
Tiny polyfill for the placeholder form element attribute. Requires Prototype.js.
Css3 Finalize
Tired of adding prefix for each broswer when it comes to the new css3 attributes?
Respond
A fast & lightweight polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more)
Yepnope.js
An Asynchronous Conditional Resource Loader
Stringformatter
JS string formatter that supports objects, currency, date/time, decimals, and more supports easy extension and garbage collection ... goes far beyond sprintf approach.
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
Store
A better way to use localStorage and sessionStorage