http://www.microsoft.com/windows/internet-explorer/beta/ (for XP, Vista and Windows Server 2008 only)

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There are certain risks to installing and using beta software on your main PC. Be sure to perform a full backup of your system (or, at the very least, your files) before installing. There know incompatibilities with some programs, one of the most notable is Windows Live Mail:

Internet Explorer 8 Beta 2 and Windows Live Mail are not compatible at this time. If you install Internet Explorer 8 Beta 2, Windows Live Mail will crash when you create or reply to an e-mail message. No workaround is currently available.

The IE8 Beta 2 Release Notes are here. Please read them before downloading IE8 Beta 2.