My general rule with finding money is that if you're able to give it back (eg- the wallet has an I.D) then you definately should....
If its just loose money on the street then its a free lunch.
But those people who dont appreciate it when you return their stuff really annoy me.....Its like dude, i just went out of my way to give you back your wallet, i'm not expecting a reward but can i get a thankyou?