I would probably keep it but it depends on the situation and how much money is involved. I think another good question might be: If you saw someone drop money without knowing it, would you pick it up and give it back to them, or put it in your pocket?
The feeling you get from giving someone back their hard earned money is priceless. At least try to give it back put up a posting in the area and then if after a couple of days no one claims it it's yours.
Aha, I picked up a quarter this morning, so of course I'd pick up $1+. Finders keepers, losers just don't get the money. xD
I found a wallet with $50.00 in it when I was a kid. I took the money and threw the wallet in a mailbox. Is that still done? If you find a wallet, throw it in a mailbox? Thats what we were told to do when I was a kid.