Microsoft released Windows 8 Release Preview & Server 2012 Release Candidate on 31st May 2012, which is the pre-RTM version of Windows Client & Server Operating systems, going to be available later this year. You can download both Client & Server operating systems using links below
Download Windows 8 Release Preview http://aka.ms/win8rp
Download Windows Server 2012 Release Candidate http://aka.ms/ws2012rc