Microsoft released the first beta version of IE8, you can download it over here.
Internet Explorer 8 Beta 1 is a developer preview for web designers and developers to help prepare their websites for the launch of Internet Explorer 8. Some of the new features designed for developers include a developer toolbar and improved interoperability and compatibility.
Internet Explorer 8 is designed to work in standard mode out of this box. However, Microsoft provides a way for users to browse the web in a way similar to Internet Explorer 7 by using the emulate Internet Explorer 7 button on the chrome.