these include the Colosseum, Pantheon, Altare della Patria(The Monument to Victor Emmanuel), The Aventine Keyhole, Sant'Agnese Mausoleum (at the catacombs), and the Trevi fountain at the bottom of the Spanish Steps.
Other sights that I highly recommend are the museums! I can't say enough that the art and culture kept in the museums of Rome are incredible. I could include so many photos of sculptures and paintings but to me there is some sort of sanctuary of seeing them in the museum!