Poly
Small, fast, awesome. The only ES5-ish set of polyfills (shims) you can mix-and-match because they're individual AMD modules.
Babel
Babel is a compiler for writing next generation JavaScript.
Store
A better way to use localStorage and sessionStorage
Css3 Finalize
Tired of adding prefix for each broswer when it comes to the new css3 attributes?
Placeholders.js
A JavaScript polyfill for the HTML5 placeholder attribute
Respond
A fast & lightweight polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more)
Es6 Shim
ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines
Placeholder Shiv
Tiny polyfill for the placeholder form element attribute. Requires Prototype.js.
Hitch
A lightweight engine that allows you to hitch new capabilities to the browser (prollyfill).
Augment.js
Modern JavaScript in your IE
HTML5shiv
This script is the defacto way to enable use of HTML5 sectioning elements in legacy Internet Explorer.
Selectivizr
Selectivizr is a JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8.
Svgeezy
Allows for automatic Image fallbacks for browsers that can't handle SVGs
Flexie
Legacy support for the CSS3 Flexible Box Model
CSS3 Pie
A behavior for Internet Explorer allowing it to recognize and render various CSS3 box decoration properties
Yepnope.js
An Asynchronous Conditional Resource Loader
CSS Parent Selector
CSS4 parent selector based on jQuery