If you're going on your own, RomaPass is the thing to get. They coaxed most of the museums into this initiative, so as a result for 18 euros you get:
- A three-day pass for Metro, buses, trains to the seaside etc (worth 11 euros)
- Two free admissions to the museums/galleries/sites (my picks would be Colosseum + Palatine - 11 euros and Borghese - 8.50 euros - you still have to pay the 2 euro booking fee).
- Reduced admission (mostly, half price) on further attractions.
- In some places (Colosseum) you skip the queue - there's a separate entry for Roma Pass holders. Can save you quite some time (like even half an hour in the Colosseum).
What I calculated was, that what I got for these 18 euros, would cost me twice as much without Roma Pass.
You can get it at Tourist Information offices and the affiliated sites (I think the Termini one and the one near Colosseum are most conveniently located). For more - please see http://romapass.it