Probably the most famous fountain in the world, Trevi was constructed in 1762 by Nicola Salvi. It is also the largest Baroque fountain in Rome with almost 80,000,000 liters of water circulating through the system every day.
You can see hundreds of tourists throwing coins into the fountain every day since it is said that doing this ensures your return to Rome one day. The "proper" way how to do this is to use your right hand and throw the coin over your left shoulder. All the money collected from the fountain is used to feed Rome's citizens in need.
Not surprisingly, this famous landmark appeared in many movies such as Roman Holiday and Three Coins in the Fountain.