Windows Server 2012 R2 build 9431

This build was released on 25/6/2013 through MSDN (screenshot). Later on, it became publicly avaliable through Microsoft Technet Evaluation Center. First it was thought to be released alongside the client build on BUILD 2013 at 26/6/2013, but it came a day earlier. On the same day, System Center 2012 R2 Preview was also released. It is the first leaked build to have the new start button.

Remote Server Administration Tools
Alongside this build, a preview version of RSAT was released. It has the same build number. Should work on both Windows 8 and 8.1.

Variants
When downloading from the Technet Evaluation Center, you can choose between an ISO, VHD (Core/GUI) and a Windows Azure version. VHDs are avaliable only in English, but the ISO was released in 12 languages: Chinese (Simplified), Chinese (Traditional), English, French, German, Japanese, Korean, Portuguese (Brazil), Russian, Spanish, Swedish and Turkish. It seems that the MSDN also contains a Debug/Checked version. While on the Technet you can download only Datacenter and Essentials, MSDN also has Hyper-V server. The EditionMatrix also lists a new SKU called "ServerCloudStorage", which wasn't avaliable in the previous Windows Server releases.

Whitepapers
Windows Server 2012 R2 and System Center 2012 R2 Evaluation Guide Windows Server 2012 R2 Evaluation Guide System Center 2012 R2 Evaluation Guide Windows Server 2012 R2 Overview White Paper