We've called these poker playing cards, but obviously they could be used for any card game.
One of the exciting ideas we came up with, was playing a game with an absent player - no longer does your regular poker night need to be disrupted because one or more players can't make it! Using these special playing cards, and a card-reading table, you can deal your friends in, and have them play over the 'net.
Anyway, that's way off in the future - we've yet to prove that the idea even works yet. But that'll be made much easier, now we've received this full deck of playing cards from Custom Stuff:
Can't wait to get making at BuildBrighton tonight, to see if we can actually read the cards using a few LDRs and a PIC microcontroller....