SS Quattro Coronati...

One of the most overlooked churches in Rome is that of SS. Quattro Coronati. This is strange given its location - it's just a few minutes walk from the Lateran Basilica on the way to the Colosseum and a few steps from the deservedly touristed San Clemente. Perhaps the slightly dingy exterior with its Carolingian tower over the gate (the only one of its type in Rome and a relative rarity outside Germany) and double courtyard are somewhat off-putting for the casual tourist.
This view from outside gives a clue as to its significance - it was reconstructed after the Norman Sack of Rome to be the only fortified abbey within the city walls. Its vicinity to the Lateran meant that the Pope could hole up there in times of crisis and indeed it was here that Innocent III received St Francis of Assisi. However, its history dates back to early post-Constantinian Christianity. It seems to have been one of the larger and grander churches (tituli) which sprung up after the legalization of Chrsitianity. It was a large private dwelling with an apsed reception room which was converted into a substantial Church and (probably) clergy residence with administration facilities attached.
It is currently occupied by enclosed Augustinian nuns (noted for their fine singing) and a new French congregation The Little Sisters of the Lamb who are part of the Dominican Order. The former occupy the church itself and most of the attaching monastery, the latter occupy the section of the monastery overlooking the outer courtyard. The Little Sisters also sing beautifully and are outstanding in their joyful witness to Christ and the austerity of their life.
Inside, the Church is quite small - much smaller than it was before the Norman Sack of Rome. However, it boasts a wonderful cloister, the head of St Sebastian (over a side altar on the left of the Church) and this astonishing 17th Century apse painting. (Click to enlarge.)
Off the courtyard, one can (by requesting the key from one of the nuns) visit the Chapel of St Sylvester with its 13th century frescos. They tell the Legend of Constantine. This one, for example, has a judgement scene, and beneath it three scenes from the Legend. At the bottom left we see the leprous Constantine about to slaughter 300 babies so that he might bathe in their blood and cure his disease. Fortunately, he is moved by the tears of the mothers and refrains from so doing. In the centre, SS Peter and Paul appear to him in a dream and tell him to approach Pope St Sylvester for help. The third scene is of him riding out to find Sylvester.

The Legend goes on to recound Constantine's baptism at Sylvester's hands and his miraculous cure. This final snap shows the Emperor leading the Pope's horse about the City of Rome.
Edited to add: For those who are wondering, the Titular of the church is Roger Cardinal Mahoney.