In the Campo de'Fiori today, the Palazzo Pio can be seen facing the Piazza del Biscione. This palace was built over the ruins of the Theatre of Pompey. If you would like to see these remains, ask at the Pancrazio restaurant.
The theater was built in 55 B.C. and was Rome's first stone-built theater. Before this time, theaters were made of wood and were considered more temporary structures. In order to make this theater permanent, Pompey built a temple to Venus on top.
To the east of this theater was the Porticus of Pompey. On March 15, 44 B.C., it was here that Julius Caesar was murdered. If you want to see this spot, it occurred at the foot of the statue of Pompey where the Palazzo Spada now stands.
If you would like to read more about this, check out the Blue Guide Rome published by Somerset Norton.