I usually leave it since it isn't mine. If I see someone drop change, and it looks like they can't easily pick it up, I will grab it for them.

I found $100 at a casino when I was a kid. I turned it in. No one claimed it, so I got it about a week later.

I use cash very infrequently, so change doesn't build up much. Pennies either get left in the store, tossed on the ground, or thrown in the trash.

I have two dimes sitting on my desk. They have been here a couple of months. The vending machine downstairs sometimes has dime sales to clear stuff out, so I keep the dimes. I would probably throw them away otherwise.