The Trevi Fountain (Fontana di Trevi) is one of the best known sights of Rome, thanks at least in part (and perhaps unfairly) to its role in Fellini's film La Dolce Vita: it was here (or rather, on a recreation on set at Cinecittà) that Anita Ekberg did her splashing about. You won't want to do the same thing yourself, however: there is bleach in the water now.
History of the Fountain
The Trevi Fountain was designed in 1732 by Niccolò Salvi, who won a competition held by Pope Clement XII. It was designed on the site of an earlier and uncompleted fountain by Bernini, and built onto the back of a newly completed palazzo. The palazzo had only just been completed by the Duke of Poli, who had designed a small recess into the building to allow for a modest fountain. Instead of using this recess, Clement XII allowed Salvi to take up the entire end of the building, brushing aside the Duke's protests. After all, he reasoned, it may have been the Duke's building, but it was the pope's water. The fountain took thirty years to complete, by which time Salvi was dead.
The Aqua Virgo
The site is historically that of the terminus of the Aqua Virgo, one of the aqueducts that supplied ancient Rome with water. The aqueduct was named after a legend that the spring which provided its source was discovered with the help of a young girl.
Some legends suggest the girl was named Trivia, but this is unlikely, as it's Latin for 'three roads', which would be an odd name for a girl. The fountain itself is probably named for the fact that it was built on the intersection of three roads.
The story of the discovery of the spring by Trivia (or whatever her name was), and of Agrippa's subsequent development of the aqueduct, is depicted in parts of the decoration. Salvi's original plans developed this theme further, with statues of Agrippa and the girl, but the plans were changed after Salvi's death.
Visiting the Trevi Fountain
Wandering around the centre of Rome, its surprising how often you will find yourself at the Trevi Fountain. You hear the rush of water first, and the hum from the crowds of tourists photographing each other, throwing coins into the fountain or defending themselves from the numerous sellers of plastic bubbleguns and strange rubbery spheres. (Watch out for pickpockets in the area.)
It's a two minute walk from the Quirinale palace, and could be the first stop on a pleasant short walk that will take you on past the Pantheon to end up at Piazza Navona; alternatively, the Spanish Steps aren't too far to the north of here.