1. Selectivizr

CSS3 selectors for IE selectivizr is a JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8.

Selectivizr adds support for 19 CSS3 pseudo-classes, 2 pseudo-elements and every attribute selector to older versions of IE.

Integration is pretty simply including the script in your pages and selectivizr will do the rest. And it also works with other JavaScript Libraries like JQuery, Mootools, Dojo and Protype etc..

http://selectivizr.com/

2. Modernizr

Modernizr is a JavaScript library that detects HTML5 and CSS3 features in the user’s browser.

Modernizr makes it easy for you to write conditional JavaScript and CSS to handle each situation, depending upon the browser supports a HTML5 and CSS3 supports or not.

Details about the Modernizr documentation: http://modernizr.com/docs/
http://modernizr.com/

3. Respond.js

A fast & lightweight polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more).

Respond.js enable responsive web designs in browsers that don’t support CSS3 Media Queries (IE 8 and earlier versions of Internet Explorer).

https://github.com/scottjehl/Respond

4. Response

Response is a jQuery plugin that helps optimize cross-device performance by enabling progressive enhancement of content via HTML5 data.

Response JS is a jQuery/Ender/Zepto plugin that gives web designers tools for building performance-optimized, mobile-first responsive websites. It provides semantic ways to dynamically swap code blocks based on breakpoints and serve images (or other media) progressively via HTML5 data attributes.

http://responsejs.com/

5. CSS Pie

You can write CSS3 without worrying about IE6 – IE9 fallback; Pie makes Internet Explorer to supported many CSS3 properties like Border-radius, RGBA, Gradient, Box-shadow etc…

http://css3pie.com/