Sometimes Free

Rome sights (er, sight) that are (is) sometimes admission-free

Unlike many other major European capitals, Rome doesn't really go in for the whole "get in free on the second Tuesday" sort of thing—aside from the annual, week-long, nation-wide Settimana per la Cultura, when all state-run sights throughout Italy are free (national museums, galleries, monuments, and archaeological sites); usually the third week of April. » FULL STORY

There is one whopper of an exception:

Rome for Free
Always Free
Sometimes Free
Churches
Markets
Parks
Experiences
Discounts

The Vatican Museums - Rome's most magnificent collections of museums—including the Sistine Chapel—are free on the last Sunday of each month. Of course, this is not a secret, so expect a crush of visitors, especially of schools on class trips and other cash-strapped groups. Sure, you get in for free, but the crowds may not make it worth your while. At the very least, try to show up when the doors open so you can be at the head of the surging masses (and make a beeline for the Sistine). » Full Story