If you're in a loving relationship, you're going to end up paying your partner for some things — maybe they ran some errands for you, bought you some new deodorant, and you want to pay them back.
If you're in the US, Venmo is one of the best apps for paying people back and forth. I used it all the time when I was living in the States.
Now that I'm living in Canada, I use Transferwise, which removes the headache of currency conversions to ensure the person you're paying gets exactly the amount they're owed, in their native currency.
Note: Both apps may require a small fee for some transfers, depending on your method of payment, so be sure to read the fine print in each app to minimize any fees.
Price: Free