The new +Dominion expansion, Dominion: Dark Ages, comes with a very clever trick presumably related to the upcoming online and mobile version. 

It includes a card which it instructs you to "hold here" and hold up to the screen of your phone.  At first, I thought this couldn't possibly make any sense: NFC isn't super ubiquitous and works on the back of phones.  It wasn't telling you to hold it up to the camera, and besides, there's nothing obviously machine-readable on the card.  The card isn't transparent and didn't look like it would "line up" with anything on screen in any meaningful way.  But, I figured they must have something up their sleeves.

Opening up +Markers, and holding the card up against the screen as directed yields 5 spots.  They have some sort of conductive element in the card creating a multi-touch interaction.  I'm guessing once the real app comes out, if you do this, it will recognize the particular spacing/configuration of this 5-touch gesture and do something interesting.  

It's clever in that it should work well across a broad variety of devices, but is hard or impossible to accidentally trigger by hand, depending how precise the tolerances are on the spacing of the 5 points.  I look forward to seeing what the app actually does with it.