Microsoft has released Windows Internet Explore 8 (IE 8) today. You can download it from http://www.microsoft.com/ie8 website or from Microsoft download section.
The new IE 8 will work on Windows XP, Windows Vista & Windows 2008. But you cannot install it on Windows 7 Beta. Microsoft planning to incorporate IE 8 into release candidate version of Windows 7.
Some of the users like me who want to test IE 8 for few days/week and then uninstall may want to check the previous post How To Uninstall Windows Internet Explore 8 (IE 8)