Windows 8.1 and Server 2012 R2 Previews

Here are some places you should be able to find the Preview downloads of 

Windows 8.1 and Server 2012 R2

Windows 8 Home users can upgrade here
http://windows.microsoft.com/en-us/windows-8/download-preview

MSDN 

(If you have a subscription) Has the Server 2012 R2 ISO now and should have Windows 8.1 sometime after 9am on June 26, 2013.

https://msdn.microsoft.com/en-us/subscriptions/securedownloads

Technet

http://technet.microsoft.com/en-au/evalcenter/dn205292.aspx

The Windows Enterprise Website

The Windows 8.1 Launch page

http://www.microsoft.com/en-us/windows/enterprise/products-and-technologies/windows-8-1/default.aspx

Enterprise ISO Download Link

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

Make sure you watch the Keynote from Build 2013!

http://channel9.msdn.com/Events/Build/2013

Windows Connect
Should you want to give Microsoft feedback on your testing, you can sign up at Connect, but I don't think they are taking 8.1 testers right now.
http://connect.microsoft.com/

Windows Store
You will also be able to download 8.1 from the Windows App store if you already have Windows 8.  There will be a notification that will show up for Windows 8 users to upgrade.
Windows 8.1 will be a free upgrade for licensed Windows 8 users.

Warning!  

You will have to wipe your hard drive when the final version of 8.1 is released, so you may want to use a test machine or backup your PC before you install the Windows 8.1 preview.  Windows Easy Transfer will allow you to backup your PC before you move to RTM, but that will not re-install your apps.