At Rome's Termini station, if you want to get a cab, make sure you go to the place where the real cabs are. My brother and I got into a cab that wasn't, because we were desperate. The guy didn't know where he was going, and we were charged 25 euros to go about 1/2 a mile. Make sure you get the right cab.
Look out for the Sunday "supplemento", acoording to one cab driver, who charged us nearly $11 for a ride from Piazza della Repubblica to Piazza Navona, a ride which should have cost around 7 Euro. The longest trip we took in a cab was from Piazza della Repubblica to Vatican CIty, cost about 8 Euro. The next day the same trip cost 10 Euro. Go figure.
Unique Suggestions: Of course, ask the price first. Cabs are not really necessary for sightseeing, so walk. Don't worry, you'll find it on your own. We walked all the way to Trastevere one morning ( extreme for some, but we did it, even with a seven-month-old).
On arrival at the Termini station you have to beware of unregistered taxis. These appear to be private vehicles whose drivers dispose of licenses to operate as taxis. Since they don’t employ a counter, you have to settle on a price before setting off. Drivers wait readily at the entrance of the Termini station to offer their services. Unfortunately they seem to luck profound knowledge of the city. Despite our efforts to explain to our driver where we wanted to go, he didn´t seem to have a clue about the fact that there are 2 Sheraton hotels in Rome let alone their location. To help himself out he drove us to the wrong hotel . The problem is that you can hardly sue him as is the case with a proper taxi. We had to pay him and call for another taxi to transport us further. Thereby we paid much more than we would have if we had driven directly to the Hotel.
Fun Alternatives: don´t give in to taxi drivers hunting for passengers in the Termini station. Avoid cars without authorised taxi-label on top.