A completely random ordering of a deck of cards. You can have a deck pre-stacked in this order, learn some false shuffles, have someone pick a card and place it back anywhere they want without marking its location in any way, and when you inspect the deck you know exactly what their card is. And they'll never guess that the way you did it was memorizing the order of every card in the deck.
I'm sure there are a lot more advanced ways to take advantage of this, just a handy ability to have in your back pocket (literally).
If you're going to memorise a deck of cards, you're better off learning something like the Mnemonica Stack as you can use it as the basis for a whole load of card tricks.