I was first introduced to playing cards in rpgs when I played Savage Worlds. In that game a deck of cards is used for (but not limited to) initiative, chase mechanics, roleplay, and random encounter generation. The experience was comparable to rolling your first non-d6 or your first zocchi die. In a way the deck of playing cards made the game feel fresh and new in a system that was marketed as generic.
Ever since that short stint with Savage World I've been obsessed with introducing the deck of cards into my games.
Here's just a few ways that I've used, or am thinking about using, cards in my games.
Straight up stolen from Savage Worlds. Deal a card to each player and group of enemies. Aces can be high or low and the highest rank goes first. Ties are resolved by suit. In Savage Worlds a player who draws a joker gets to go whenever they want during the round and gets an additional +2 to their rolls. (This is also the calling card for the referee to shuffle the deck.) You don't have to do that or you could take it one step further. Maybe players get +2 to their next attack-damage roll if they draw a king. Likewise a queen could allow for a temporary parley to negotiate.
Sometimes you don't have dice just laying around. Maybe you've started a pick up game with your customers at work or you've introduced resolution systems to a lost tribe in the amazon. Anyways, you don't have your dice but for some reason you've brought your trusty deck of playing cards! Needs a d27? No prob! Just make a mental layout of each card's value, shuffle, and draw! The numbers work out quite well. I once made a 2d6 out of a deck of 44 and out of 60 results the average only deviated by 0.0166! Math aside, the more cards in your deck the more random the results will be. For example, you'd probably want to shuffle 4 suits of 7 cards each if you're making a pp7. That's prison polyhedral seven. In fact I'm declaring the short hand for all card-related number generation is now pp! pp8? pp10. pp37!
Sometimes the idea of rolling 18d6 is scary to a new player—even if it's devided into 3d6 chunks—or your players are just lazy like mine. What to do? Simply buy blank playing cards, generate your own stats, and write them on the cards with markers! It's sounds like a lot of work at first but if you have access to an online dice roller (and who doesn't these days?) the process is made even easier. It will pay off in the long run because you can tuck the card back in your deck for a rainy day when you or someone you know needs a quick character. You can go a step further and include backgrounds, class, or maybe a few roleplay ques. Which brings me to my next use.
I once shuffled a deck of face cards and laid them on the table in a grid. I told the players that each card had an adjective tied to it that would describe their character, but didn't tell them what they were. They took turns picking cards and I would tell them their adjective afterwards. After all the cards were gone and the game started, roleplay was a breeze! Keep in mind this was for a group that had trouble with roleplay. At the time I was honestly surprised it worked so well, but after asking the player what they thought, they liked it because "the character was already made for them!" Honestly I think a deck of tarot cards would work much better but my knowledge of their use isn't that great so I'll stick to regular playing cards!
That about wraps things up. If I discover yet more interesting or effective uses for the deck of cards, I will either update this post or make a new one.