Microsoft Windows 8 Pro Pack @ Microsoft Store. It's a special pre-order price that will go up. Note that you do not need this if you just bought a Windows 7 machine since you will qualify for the $14.99 upgrade pricing.

But since I'm running a MacBook Pro with Parallels, I'll need to order this to run on my Mac - It's probably the best price you will see for a while on Windows 8 Pro with Media Center.