Villa Pamphili, Rome's secret park.

The Eternal City's largest park, with landscaped gardens and lots of green space.

Why visit? 
A visit to Rome can be exhausting, given the summer heat, baroque overload, tourist stampedes and the mock gladiators browbeating you for pictures.  Most visitors at some point end up in the well-known Villa Borghese, which is still crowded and where itinerant balloon salesmen descend like vultures upon families with kids. Villa Pamphili is Rome's other park, where Romans go to run, play and relax, free of tourists and touts.

Villa Pamphili is literally off the map. Most tourist maps of central Rome barely show a tip of the park, just behind the Gianicolo. In fact, Villa Pamphili is walking distance from central Rome, in the Monteverde neighbourhood, a stroll up the hill from Trastevere. Map

The main entrance near Porta San Pancrazio. 
LIKE MOST THINGS in Rome, Villa Pamphili (officially called Villa Doria Pamphili, sometimes spelled Villa Pamphilj) has a long and illustrious history.

Back when the city of Rome stopped at the Aurelian walls (you will have passed through them at Porta San Pancrazio), the noble Pamphilj family decided they needed a little country house. (The family also had palaces on Piazza Navona and Via del Corso- I guess even back before the days of mass tourism the center of Rome could feel rather crowded.)

It started off as a little casino (small country house) but when Giambattista Pamphilj managed to get himself elected Pope Innocent X in 1644, clearly such a humble dacha was insufficient for the successor to St Peter. (Being a servant of God rarely stopped Popes from desiring a bit of bling and luxury to pamper themselves.)

(As an aside, you may have seen Innocent X's face before- there's a famous portrait by Velasquez, and Innocent received a second round of posthumous fame when Francis Bacon painted him as the Screaming Pope)

A new villa was built and this is the one that still stands today at the heart of Villa Pamphili. Although built when Baroque was very much the flavour du jour, the villa is more Mannerist/Renaissance in style, and is still known as the Casino (but don't get too excited- there's no Black Jack or roulette in sight.)

Of course, what use is a country villa if you can't go for a stroll? The building is surrounded by a 'secret garden', manicured grounds, lemon trees, a few follies and a (now defunct) water grotto. 
Over the years, the Pamphilj descendants (now called Doria-Pamphilj after a successful merger with another aristocratic family facing slow decline, hence the official name of the park) extended the grounds. Later on, bits were acquired by the City of Rome and the Italian state and since 1972 the park is open to everybody, noble or not. 
The Secret Garden is hidden behind a wall- unfortunately it's so secret that you can't go inside. The Casino is closed as well. 
Villa Pamphili is now Rome's largest park but largely overlooked by tourists. True, you can't visit the Casino (unlike at Villa Borghese) but the park offers Romans and visitors alike some fresh air and open space. The park consists of two parts with a bridge connecting the two halves. 
Jogging and cycling are both popular activities. Lazing about, dolce far niente, is too. 
Dolce far niente next to an egg on a pillar(?!)
Just west of the Casino there are large lawns which during weekends become football (soccer) and ultimate frisbee fields. And there are restaurants and children's play areas near the Via Vitellia/Via Leone XIII entrance. 

Tall umbrella pines are a distinctly Roman feature of the park. 
For the history buffs and engineering freaks among us: the northern wall of Villa Pamphili follows the Via Aurelia Antica, the road the ancient Romans built to Pisa (not for tourism, as this was well before the Leaning Tower: it was an army road).  On the city side of the the park, the Aurelia is flanked by the Aquaduct Traiana-Paul, first built by Emperor Trajan in the first century AD (and renovated by Pope Paul V in the 17th.)
Joggers follow the Aquaduct Traian-Paul. 
Come summer, Villa Pamphili becomes an open-air theatre with concerts and performances taking places under the tall umbrella pines. If you're around this time of year, try to attend one of the performances under the stars. 
Open-air concerts take place in front of the Palazzo Corsini. 

Getting there: 
Walk up from Trastevere to the Gianicolo using the steep steps of Via di Porte San Pancrazio, stopping for the breathtaking view of Rome at Pope Paul V's marble fontanone (big fountain). Continue towards Porte San Pancrazio, a massive stone arch, and continue a little further. Map. 

For those who are tired of walking, several buses pass the main entrance, including 710 and 870.

Useful links:
Official Website
Villa Pamphili walking tour