Microsoft announced availability of Internet Explorer 8 Beta 1. This version is primarily for web developers to test new technologies available in IE8. Also it is great to check your existing sites against IE8 new rendering engine. However, nothing stops anybody from testing new IE.
There are versions available for Microsoft Windows Vista Service Pack 1 (SP1), Windows Vista, Windows XP Service Pack 2 (SP2), Windows Server 2008 and Windows Server
2003 Service Pack 2 (SP2).
You can select your version on this page: http://www.microsoft.com/windows/products/winfamily/ie/ie8/readiness/Install.htm
More on new features here: http://www.microsoft.com/windows/products/winfamily/ie/ie8/readiness/NewFeatures.htm