Well, personally I found the following:
>> a wad of $10 & $50 bills dropped from someone walking in front of me on an overhead bridge when I was in Primary school - I picked up the wad of cash, ran up to the man and gave it all back to him... and later felt I could have done otherwise...

well.. I was young...
>> Found a wallet containing money and ID cards along with other personal effects in the toilet of a shopping center; I submitted the wallet to the information counter, and demanded that they get the owner to contact me when he collects it to make sure it was returned.
>> got into a cab after a rather rude Mandarin speaking lady alighted and slammed the door on me, and discovered she left behind her purse containing keys and over $200; I kept the money and threw away the keys... she dserved it and there were no indentity card inside.
>> found an entire clutch bag near a traffic junction while driving; it belonged to a Malay dispatch rider, whom I personally drove to his residence in Teban Gardens and handed it over to his wife. There was not much in terms of money in the bag, but it contained the man's Quran book. That in my opinion, is VERY important.
Over the decades, have picked up various denominations in money that people dropped, but no way to trace ownership. So pocket them lah...

hmm.. come to think of it, someone even paid for my petrol at one Shell station and drove off, and the cashier said she paid for me! I was so stumped..!