On the street
If you ever stumble across a $5 bill and take it from the sidewalk -- perhaps after a shuffle of the shoe to move it inconspicuously away from onlookers -- you may have broken the law.
Many cities and states (among them California) have laws requiring that all found property be reported to police and a reasonable effort put into finding the unlucky owner. The laws typically don't define a value so, technically, even a quarter dropped by the gumball machine would warrant a trip to the station.
If you kick a can littering your path, be aware that whoever failed to recycle might as well have tossed their money on the sidewalk. A nickel here and there may not seem like much, but it adds up when thousands of people in bottle bill states avoid bringing their empties back to the store or a redemption center.
In Massachusetts -- a state looking to extend its 5-cent deposit to bottled water and tea drinks -- the new fiscal year includes $20 million in revenue collected from unreturned bottles. New York, by comparison, nets more than $100 a year from unredeemed containers.