The Testaccio is a working class district near central
Rome, filled with small shops and a bustling market selling everyday things like meat and shoes, not tourist trap stuff. It is on the far side of the
Aventine hill from the “old ruins” part of Rome and just across the
Tiber from
Trastevere, a similar funky district.
It is named for the hill in one corner, now plant-covered. This is a goodly-sized hill, but not one of the Seven; in fact, it is The Hill that Rome Built as opposed to a Hill That Rome Was Built On. It is made up of shards of the old pots the Romans used to transport just about everything: the Testaccio in ancient Rome was the dockyard area, home to the original Emporium. Originally the traders just dumped their broken pots in the Tiber. After a while, this rather clogged up the river, so the shards were dredged out and piled on the bank. Unfortunately, the hill is closed to visitors now: too many shard-thieves.
The Testaccio is now also the up-and-coming entertainment district of Rome, with lots of restaurants and clubs all within an easy night-time walk.
Beware, however: since this was the meat-cutting district, the restaurants tend specialize in innards. It is the only place I have ever seen “beef nerves” on a menu.