[Last updated with prices, etc. in October 2015...the 3-day pass *used* to be €30 and thus a better deal, but as of 2014 it costs more]
Rome has the "Roma Pass", a 2- or 3-day transit and museum pass marketed to tourists as an easy and economical way to see the Eternal City...but is it worth it?
Let's break it down:
There are two Roma Pass options: 3 days for €36 and 48 hours for €28. The first gets you into the first 2 museums/sights for free, and the second one gets you into 1 museum/sight for free. (Most major museums in Rome are included, except the Vatican Museums/Sistine Chapel.)
Most people will want to use the pass for the Colosseum/Roman Forum (€12 combined ticket) and/or the Galleria Borghese (€11-13 ticket, depending on if there's a special exhibition or not).
Even assuming the most expensive Borghese tickets (€13 each), the 3-day pass would then be worth €12 + 13 = €25 which means that you're spending €11 for a 3-day transit pass. Since a "BIT" one-way transit ticket costs €1.50, you'd therefore have to ride transit at least 8 times in those 3 days to make it worth your while. Most people won't.
The 48 hour pass is an even worse deal. If you use it for an otherwise-€13 Borghese ticket, then you are paying €15 for a 48-hour transit pass, which means you have to ride transit 11 times in 2 days to make it economical.
(Oh, and keep in mind that the Roma Pass's transit benefit does not cover more expensive journeys like going to Tivioli or the airport—it only works where a normal one-way ticket (BIT) would work. There used to be a Roma è Piu pass that would cover public transit in the entire region of Lazio, but it isn't sold anymore.)
Bottom line: This pass is almost always a bad monetary deal
1. Skipping the Colosseum Line
The Roma Pass does let you skip this very long line (for buying tickets). However, there are other ways of skipping the ticket-buying line (see this Fodor's article, under "Tips"), such as buying tickets online, buying them down the street (at the Palatine Hill), or going with an organized tour group.
2. Lots of Museums
Since the Roma Pass gives you discounts at other museums (beyond the 1-2 free ones), if you want to pack in a lot of museums, it might be worth it. However, most tourists simply won't have the stamina for the number of museums it would take to make this a good deal.
Some people just want to buy 1 thing and be done with it—nothing more to worry about or lose. If you're one of these people, it might be “worth it” for you to buy a Roma Pass to avoid stress, even though you'll be spending more money than if you bought individual transit tickets, etc.. However: keep in mind that if you want to use the Roma pass to see the Borghese Gallery, you cannot reserve your spot online—you have to call (see the main Rome page for more details).
If you have young children, you definitely don't want to buy them a Roma Pass. For example, children under 10 years of age travel for free on public transit. Many sites also provide discounted tickets, while the Roma Pass costs the same for each person.
Note: a lot of other websites are spreading frankly false information about the Roma Pass, or are out-of-date. For example, this Budget Travel article incorrectly states that Roman Forum tickets are "$24" (??), and in any case compares the pass to buying a 3-day transit pass (not the one-way tickets most people would actually buy).