Windows Server 2008 R2 Beta.
Microsoft Windows Server 2008 R2 will be the next version of the Windows Server operating system from Microsoft. Building on the features and capabilities of the current Windows Server 2008 release version, Windows Server 2008 R2 allows you to create solutions that are easier to plan, deploy, and manage than previous versions of Windows Server.
Check out what's new for this release (Hyper-V Live Migration, Direct Access, Branch caching, Cluster Shared Volumes, VDI Features, Improved power consumption, ...)
The reviewers guide can be found here : http://download.microsoft.com/download/6/E/3/6E3E4529-27E3-48EC-B7E7-1A93242D3AE1/Windows_Server_2008_R2_Reviewers_Guide_(BETA).doc
Download the bits here : http://www.microsoft.com/downloads/details.aspx?FamilyId=85CFE4C9-34DE-477C-B5CA-75EDAE3D57C5&displaylang=en
