In addition to the traditional three-day Roma Pass, there is now a 48-hour Roma Pass.  

The three-day Roma Pass currently costs €36 (April 2014). and gives you two free admissions to all national and city museums in Rome, discounts to other covered sites after you've used your two free entries, and includes bus, tram, metro, and urban trains, for stops within the city boundaries.  It's valid for three calendar days, starting when you first use it (either for transportation or for a free entry) and it expires at midnight of the third day. It's usually best to begin using the pass early on the first day, because if you first use it at 10 PM, your first day will only be two hours.

If you visit two of the more expensive covered sites and use public transportation fairly often, or if you visit three of the covered sites and use public transportation a little, you'll probably save money with the traditional Roma Pass.

The 48-hour Roma Pass costs €28 and is good for 48 hours from the time you first use it for transportation or a touristic visit. It has the same transportation benefit as the traditional pass, but gives only one free entry, plus the same discounted entries as the traditional pass. Note that the traditional pass is based on calendar days, but the 48-hour pass is based on the clock. So you could use it one afternoon, the whole next day, and the morning of the third day.

It's harder to save money with the 48-hour pass than with the traditional three-day pass. If you visit one of the most expensive museums, and make three reduced-entry visits, and also use public transportation frequently, the pass may save you money. It's rather hard to fit all that into 48 hours, though. 

The Roma passes do not include travel to the airports, which are outside the city limits. They don't cover any private museums, such as the Vatican Museums, the Doria Pamphilj Gallery, the catacombs, or the Villa Farnesina.

There are a few museums that are always free with the passes even if you've already used your free entries.  You also get discounts on certain tours and activities, as well as a map and a guide to the attractions. 

For more information about the passes, including an FAQ, visit their website.  

There are some circumstances when you shouldn't get a Roma Pass. Since children under 18 get free entry to all national and city museums, and children under 12 ride free on public transportation, it doesn't pay to buy a Roma Pass for a child. Disabled people and a companion also get free admission if they have proof of disability; see the FAQ listed above for details. All national museums (but not city museums) have free admission on the first Sunday of the month, so if your visit includes that date, it may not pay to get a Roma Pass. Many museums have free or greatly reduced admission on European Heritage Days, whose date varies by year, but it's usually in early autumn. On national holidays, especially Christmas, New Year's Day, and May 1st, museums may be closed, and most museums are closed on Mondays, so you have to consider whether you'll get the full benefit of a pass if your visit includes a Monday or is during a holiday period. The clock keeps ticking on days when almost everything is closed, so check the hours of the museums you want to visit. 

The following checklist will help you decide whether the cost will be worth it to you.

1. How much will you use public transportation? Without a pass, a single ticket costs €1.50 and is good for 100 minutes including transfers (but with only one segment in the metro system). If you think you'll make two such trips a day, you can estimate that your transportation savings would be €3 a day. You can get a one-day pass for €6, or a  three-day transportation pass for €16.50, but very few people use public transportation enough to justify the expenditure. Most people end up walking a lot. Some people take taxis a lot. Some people find the bus and metro system too complicated. You are the best judge of how much you're likely to use public transportation. 

2. Which museums and archaeological sites do you want to visit? Here is a list for all the covered museums and archaeological sites, with the normal full cost and the reduced price that applies after you've used the first two free entries:

For the free entries, you'll save the full price of the ticket. For any other sites you can squeeze into the time the pass is valid, you'll save the difference between the full price and the reduced price. Note that some multiple sites count as a single site, so you can often visit more than two museums or archaeological sites free. For example, the Colosseum and the Roman Forum/Palatine Hill are two sites, but they count as one, and can be visited on two consecutive days, as long as the pass is still valid, and still leave you another free entry. The National Museum of Rome is four separate museums, but they count as one entry, which you can visit over three days, as long as the pass is still valid. You have to do the math to see if the pass will save you money, and remember that zigzagging around the city to save money wastes your precious vacation time. 

3. Add up your estimated savings on entries and your estimated transportation costs. If it's at least the cost of the pass, it's definitely worth getting it. If it's just a little less, it's still worth it for the convenience. If it's considerably less, you're probably better off not buying it. 

Some people buy the Roma Pass just to "skip the lines". This makes no sense. The only places that have long lines are the Vatican Museums, which is not covered by the pass, and the Colosseum. You can skip the lines at both of these sites by buying your tickets online. 

Don't let the pass decide what you want to see and when you want to see it. Decide first what you want to see, and whether you really want to squeeze it all into three days, and then figure out whether the Roma Pass will save you money.