A better way to use localStorage and sessionStorage
Allows for automatic Image fallbacks for browsers that can't handle SVGs
Legacy support for the CSS3 Flexible Box Model
Tiny polyfill for the placeholder form element attribute. Requires Prototype.js.
CSS4 parent selector based on jQuery
Small, fast, awesome. The only ES5-ish set of polyfills (shims) you can mix-and-match because they're individual AMD modules.
An Asynchronous Conditional Resource Loader
A behavior for Internet Explorer allowing it to recognize and render various CSS3 box decoration properties
A lightweight engine that allows you to hitch new capabilities to the browser (prollyfill).
Tired of adding prefix for each broswer when it comes to the new css3 attributes?
JS string formatter that supports objects, currency, date/time, decimals, and more supports easy extension and garbage collection ... goes far beyond sprintf approach.