Li Rioni is nearby Il Colosseo and quite good. Here is a review "The Guardian" did awhile back.
A small wood-burning oven at the back of the pizzeria turns out hundreds of thin crust Roman-style pizzas every night. This busy neighbourhood pizzeria located just a few blocks from the Colosseum sees its share of tourists early on, but from 9pm until midnight hungry locals show up in droves and queue for tables. In the summer there is outdoor seating, while the year-round seating inside is set in a room decked out like an old Roman street scene. Folklore value aside, the pizza at Li Rioni are consistently good, particularly the sausage and mozzarella pie, and the margherita is among the best in the centre of Rome. Due to its popularity and the small size of its oven, service can be quite slow at peak hours.
• Via dei SS Quattro 24, +39 06 704 50605. Closed Tues