If Rome doesn't lack for one thing, I'd say fountains would be it. There's literally thousands of fountains located everywhere. Some are decorative, some are monuments to people or Popes, some are for drinking, some are even called "Talking Statues". It's a subject a bit big to tackle in one blog, so let's just cover some basics:

In the times of the Roman Empire, around the 98 AD mark, there were 9 aqueducts around Rome which fed all of its fountains and public water supply. At this moment in time the major monumental fountains were even connected to more than one aqueduct, to ensure water was always flowing. These were among the many things in Rome which were neglected and damaged during the time after the fall of the Empire. It wasn't until the 14th century when Pope Nicholas V decided to revamp the ruined city, which included fixing up the water system that had been left to dry for so long.

One of the first Renaissance era fountains was/is the one located in the piazza in front of Santa Maria Trastevere.

In the 17th and 18th centuries the Popes had decided to fix up the other dried aqueducts, and this gave rise to a golden age of fountains for Rome. You can see the emergence and glory of the Baroque period of art and architecture strongly in the fountains from this period.

Some examples of fountains from this "golden age" are the fountains in Piazza Navona (including the central one of the Four Rivers by Bernini, completed in 1651), the Trevi Fountain (1730), and the Triton Fountain in Piazza Barberini (1642, Bernini).

Originally, how did they work? They were all masterfully designed to work off water and gravity only. They calculated how far away the aqueduct feeding it was to understand how the water would flow. Nowadays mechanical pumps help them out in performing their spectacle, though at least some of them are still fed by aqueducts.

There are something around 2.500 of small fountains around Rome alone, called "Fontanelle". They are more or less distributed in all streets and piazza's in the city. The most common type you will see has the endearing nickname "Nasoni" (like "big nose"), for it's Gonzo-esque nose as you might have guessed. Don't be afraid to use these, as this is easily the cleanest water in Rome (and that includes my water in my apartment and your water at the hotel!)

Modern day pranks: October 2007, a group of arty-activists made a protest against the expenses for the Rome Film Fest by turning the Trevi Fountain red-carpet-red. The dye did not harm the fountain at all, and leaflets were left nearby the crime scene explaining the message that was meant to be understood. This amazing photo below was snapped by Antonio Amendola.