Windows Azure    (30-day trial as well as "Pay-As-You-Go" Subscription)

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200256692

Windows Server 2012 R2

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200256993

System Center 2012 R2 

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200256991  

BizSpark Signup

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200257012

Hyper-V Server 2012 (Free Version)

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200256995

Windows 8.1

http://windows.microsoft.com/en-us/windows-8/download-preview

Windows 8.1 Enterprise

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200359358

DreamSpark Signup

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200257013  

Microsoft Virtual Academy

http://www.microsoft.com/click/services/Redirect2.ashx?CR_CC=200257009