Last night Microsoft has released the Release Previews (Release candidates) of Windows 8 and Windows Server 2012. These releases are feature frozen which means there will only made bugfixes before the RTM will be released in August.
Windows Server 2012 Release Preview: http://technet.microsoft.com/en-us/evalcenter/hh670538.aspx
Windows 8 Release Preview: http://windows.microsoft.com/en-US/windows-8/iso
How can you test these releases? I've wrote a article a few months ago on how you can test the Consumer Preview, just use the Release Preview media and this post: http://www.robvanhamersveld.nl/2012/02/29/here-it-is-windows-8-consumer-preview/