A deck of playing cards has to have one of the highest density of "usefulness" an object can have that I can think of. I would love to hear if anyone can think of any one thing nearly as useful in so many ways.

Entertainment of course - single and multiplayer, chance and betting, mathematics of all kinds, throwing, building (like a house of cards). I thought it was so cool when a character in Cryptonomicon used it to create an encryption cypher from jail.

A sufficiently randomly shuffled deck of cards is almost certainly in an arrangement that has never existed before and will ever exist again.

52 cards divides evenly by 2, 4, 13, 26. Add two jokers and you get 3, 6, 9, 18. Remove the faces and you get even more.

Its just so well designed - or evolved as it were. If you could only have one object for the rest of your life to entertain or educate you, I dont think you could do much better than a deck of cards.