I have always been quite an honest person, so I would usually take it back. I have found wallets etc and returned them or handed them in to the police. Though I found one in the restroom at a club and handed it in…then the owner who claimed it tried to accuse me of stealing money out of it.
I did once find an envelope on the ground with $200 in it and no identification of any sort. I admit that I kept that. I also once found a $20 used as a bookmark in an old book I bought from a second-hand book store. I kept that also.