Microsoft has just released preview version of Windows Server 2012 R2 & System Center 2012 R2. Preview versions can be downloaded from link below
http://technet.microsoft.com/en-US/evalcenter/dn205292?WT.mc_id=Blog_SC_TEE_WS12R2
along with above mentioned products, there’s also preview versions available for following products
-Windows Server 2012 R2 Preview on Windows Azure
-Windows Server 2012 R2 Essentials Preview
-SQL Server 2014 CTP 1
-Windows Azure Pack
For information relating to what’s new on Windows Server 2012 R2 use below link
http://technet.microsoft.com/en-us/library/dn250019.aspx
For more information relating to what’s new on System Center 2012 R2 use link below