[Error: unknown template qotd]For small amounts of money, which covers anything around twenty dollars and lower, I'd pocket the money and spend it at the next opportunity.
(I rather enjoy
Blizzards, and there are several in the areas I frequent, so it's a pretty tasty choice)
Past that, I'd probably take some photos of where the wad was sitting, then turn it in at the nearest police station.
Currently, they have a fairly nice system set up where they advertise its recovery in the paper for two weeks, and if it hasn't been validly claimed afterwards, the person who recovered it gets to keep it.
(note, however, that if it's found in someone's wallet instead of, say, with a rubber band around it, even more effort will be expended to find the owner)
If I got to keep it after the two weeks, I'd probably put up to half of it in my
Tax-Free Savings Account, then spend the rest on stuff I need and/or could really use.
(however, it's sometimes surprising to find out what I can really use versus what you'd
think I could really use...)