The famous rite of throwing a coin in the fountain has two main explanations. First, the Ancient romans used often to throw coins in fountains, rivers or lakes to make the Gods of water favor their journey and help them go back home safely. Second, this tradiition has been invented to raise funds for the maintenance of the fountain.
The Trevi fountain project was financed by the revenues from the re-intruduction of the Lotto game in Rome. The numbers of the first extraction on February 14th 1972 were 56, 11, 54, 18, 6.
Another legend says that not only who trows a coin, but also who drinks the water of the fountain can be sure to come back to Roma. Today the water of the fountain is recycled but you can still drink from the cast iron pipes located on the steps. Emprees Charlotte of Austria, wife of Emperor Maximillian celebrated the rite using a precious cup donated by Pope Pius IX.
The Trevi Fountain appears in the movies Three Coins in The Fountain (1954) and La Dolce Vita (1960) (if you know more movies post the names in the comments section).
Once upon a time when a lad had to leave Rome was brought by his fiancee to the fountain and made drink the water with a brand new cup. After the rite they broke the cup to symbolise the indissolubility of theri love.
The movie "Three Coins in the Fountain" (1954)