If you're going to stay in Rome any longer than a day and are keen on some sightseeing, the Roma Pass will come in handy. If you're on a shoestring budget it's an absolute must.
What is it exactly?
In a nutshell, it's a prepackaged deal that includes a Roma Travel Pass, which is basically a 3 day travel card (or a BIT if you prefer: biglietto turistico integrato), a Roma Pass: a card allowing you to enter for free to the first two museums (or archeological sites that participate in the promotion) and to get a reduced entry price to others you'd like to visit. The reduced rate is the EU student / senior citizen rate and is usually about half of the full price ticket.
How do you use it?
It's simple: to use the Roma Pass, just give it to the cashier at the ticket office of the first site. You will get a free ticket, and the Roma Pass will be good for three days, including the day of your first entrance. To use the Roma Travel Pass, fill out your name on the back and validate it on the bus or at the metro station as you would any other travel card. It is also valid for three days, until the midnight of the third day.
Where can I get it?
You can get it at any participating museums or archeological sites, among them: The Colosseum, Castel Sant'Angelo, Borghese Gallery, as well as at the tobacco stores at the Termini Station, Leonardo Da Vinci (Fumicino) airport, or at any of the tourist information (IT) points around Rome.
How much is it and is it worth it?
Roma Pass costs €20. Whether this is worth it to you depends on how long you're staying in Rome and what you're planning to do. Let's do some simple math:. you're in Rome for three days, want to see the Colosseum, Forum Romanum, Saint Peter's, Castel Sant'Angelo and Borghese Gallery. You're on a budget and not planning to use the taxis. Above is pretty much the standard itinerary for a three day affair.
Without a Roma Pass: you're looking to spend €11 on a 3-day travel card, pay €13 for the Colosseum (with this ticket, you get in for free at the Forum Romanum), get in for free to Saint Peter's, pay another €7 for Sant'Angelo and €10.50 for Borghese Gallery. Total Damage: €41.50.
With a Roma Pass: you buy the pass at €20, get the travel card for free. Visit the Colosseum (and the Forum Romanum) as well as the Borghese Gallery for free. Visit Saint Peter's, then go and pay reduced price (€4 for the Castel Sant'Angelo). Total Damage: €24.
Do your own math, but as I stated above, as long as you're staying more than a day in Rome and plan on sightseeing, this is a great deal.
For more information check out Roma Pass's web-site