Halo: Spartan Assault [Trailer]
A top down, twin stick shooter that takes place in the Halo universe for Windows 8 and Windows Phone 8 with full on XBox Live integration including achievements and bonus content for Halo 4.
I think it looks pretty cool, plus a bonus, if you don't like touch screen controls you can use a mouse and keyboard to play the game (sadly no controller support).
My only criticism of this so far is one with the Windows Runtime (the fancy stuff that powers the new apps) and it's that this game is can't be one universal app for Windows 8 and Windows Phone 8.
Pretty much Windows 8, Windows Phone 8 and the XBox One all provide the same platform to run apps but the three operating systems are forced to run separate apps for no reason at all.
So if you have both Windows 8 and Windows Phone 8 and want to play this game on both, you'll have to pay for it twice.