One time there was a police SUV that made a turn onto some street that my girlfriend and I were walking on and a wallet just FLIES OUT of the car (how the fuck did that happen?), and it was a police officer's wallet with a bunch of cash, credit cards, identification so I was pretty scared of NOT giving it back. Anyway, I went to the police station, gave it back, and they made me leave my phone number. Next day I got a call and the police officer thanked me and bought me dinner It was pretty awesome.

I would cut it up. I bet the police would, too.

Those guys are giving some good advices, I would ut it up too..Who knows, it might have a virus on it ready to be unleashed...

Unless it is a really old credit card (like one of the original AMEXs, those are awesome) just cut it up. Otherwise, it might have some sentimental value or some shit, who knows.

For shit like a full-blown wallet, though, it's a pretty dick thing to do to not return it. One card is not much work to get replaced, but your driver's license, all your credit cards, your debit and any other related cards (like points cards or gift cards) would take forever to get back up to speed. I work in retail and a lot of people keep all their receipts in their wallets too (I don't know, I guess if they have to EMERGENCY RETURN something) and those can't be replaced, so anyone who just picks up a wallet, dumps the cash and keeps going is a grade A asshole in my opinion.