Go to the Trevi Fountain in Rome and toss a coin into the water with your right hand over your left shoulder. You'll have good luck. You'll also help feed the poor in Rome, as all of the money collected from the fountain is used for that purpose.
What about other fountains? Adam Chandler of The Atlantic investigated what happens to coins thrown into them. In New York City, the maintenance workers or the city government sometimes keep it. In Kansas City, homeless people usually retrieve the coins. Private fountains, though, such as those in malls, often donate the money to charities. For example, the Mall of America in Minnesota gives the $24,000 it collects from fountains every year to non-profits.
-via Lawrence E. Forbes