Microsoft just announced the availability of Windows 7 Beta which Is the next generation Microsoft Client Operating System, and also Windows Server 2008 R2 beta which is the next version of the Windows Server operating system from Microsoft built on Windows Server 2008
Windows 7 Beta or Windows Server 2008 R2 Beta can be downloaded using a TechNet Subscription or a MSDN Subscription from today onwards
More information on Windows Server 2008 R2
http://www.microsoft.com/windowsserver2008/en/us/r2.aspx
More information on Windows 7