IE7 is a JavaScript library to make Microsoft Internet Explorer behave like a standards-compliant browser. It fixes many HTML and CSS issues and makes transparent PNG work correctly under IE5 and IE6.

This is great. By adding the ie7.js script to your site code, you effectively force all older browser versions to function as IE7 does. Now that IE7 is out, there’s a second script (you guessed it: ie8.js) that brings IE7 clients almost completely up to standards-spec.

If you’re a web developer, you know how frustrating it can be to get things to display correctly in all browsers. Instead of resorting to trial-and-error methods and applying various random hacks to make things work, these javascript libraries have condensed all of the necessary hacks into a single library, so you can code normally and pretend they don’t even exist. As MS releases product updates, you can get the latest javascript library and put it in place without too much worry about having to tweak your site to make it render properly in newer browser versions.

ie7-js: Make MSIE behave like a standards-compliant browser – Link