I've got one. I actually came up with this on my own, but I'm gratified to see that EY has adopted it

cashback credit cards. these things essentially reduce the cost of all expenditures by 1%.

...but that's not where they get munchkiny. where they get munchkiny is when you basically arbitrage two currencies of equal value.

as a hypothetical example, say you buy $1000 worth of dollar bills for $1000. by using the credit card, it costs $990, since you get $10 back. you then take it to the bank and deposit it for $1000, making a $10 profit. wash, rinse repeat

the catch is, most of them have an annual fee attached, so you it's a use it or it's not worth it scenario (note, though, that for most people, if they use it for rent and nothing else, they'll save about the same as the annual fee). also, most of them need good credit to acquire, so if you're a starving college student with loans, kiss that goodbye. also, you cannot directly withdraw cash and get the 1%, so you have to come up with a way ton efficiently exchange a purchasable resource for money.