The Quirinale Palace (first of the Popes, then of the King of Italy, now of the President of the Republic). 
To the left you can see the Gardens (refer also to the panoramic photo). To the right you can see the Torre Spaccata (Broken Tower).

The Vittoriale, or Altare della Patria 
(Altar of the Fatherland). 
The monument celebrates the unification of Italy occured in 1870. The equestrian statue (look) is of King Vittorio Emanuele II. It also hosts the unknown soldier of WW1.

From left to right: the "Gazometer" of the quarter Testaccio, the Aventine Hill, the Dome of St. Paul Out of the Walls. 

From left to right: the Dome of the Synagogue, and the dome of the Church of Jesus (Chiesa del Gesu'), the main church of the Jesuit order.

The Church and the Belfry of Sant'Andrea delle Fratte.

The Church of the Collegio Romano (Sant'Ignazio), and the Dome of the Church of San Carlo ai Catinari. In the background the Janiculum hill can be seen.

From left to right: the dome of the Church of the Trinity of Pilgrims, the Column of Marcus Aurelius, the Church of Sant'Andrea della Valle (behind it), the Dome of the Pantheon. In the background you can see the Janiculum Hill, with the Church of San Pietro in Montorio, and the Paolina Fountain (it is the one with many arches).

The Dome of the Pantheon (in the background San Pietro in Montorio and the Paolina Fountain), and to the right the Spiral Dome of the Church of San Ivo alla Sapienza by F. Borromini.

The Palazzo Montecitorio (seat of the Italian Parliament), and behind it the Dome of the Church of Sant'Agnese of Francesco Borromini in Piazza Navona. In the background the Janiculum Hill, with the Equestrian Statue of Garibaldi (the pedistal singles it out).

(Still from left to right): The "New Church" (Chiesa Nuova), and next to it the Dome of the Oratory of the Filippini by F. Borromini. Next is the sharp belfry of the Church of Peace (Chiesa della Pace), followed by the Clock Tower (Torre dell'Orologio) by F. Borromini. In the background still the Janiculum. What looks like a column is the Lighthouse (Faro) of Mancini (1911, donated by the Italians of Argentina).

The Domes of Sant'Augustin (front) and of
San Giovanni Battista dei Fiorentini (behind it).

The Church of San Carlo e Sant'Ambrogio al Corso. Behind it you can see the Vatican Walls, encircling the Vatican Museums.

The Palace of Justice (Palazzaccio), and behind it the Basilica of St. Peter with the Dome by Michelangelo, and the Vatican Palaces, residence of the Pope and of the highest prelates.

The hill of Monte Mario, with the Hilton Hotel

From left to right: the Church of the Trinity on top of the Spanish Steps, the Hassler Hotel, and the Intercontinental Hotel.

From left to right: Borghese Gardens, and the Villa Malta, administered by the Church.