Porta Portese is the massive flea market that takes over Trastevere every Sunday from 7am to 2pm. The vendors have a wide variety of goods, including antiques, coins, books, furniture, clubbing clothes, fake designer purses, home decor items, jewellry, and a sprinkling of everything else.  Most vendors are willing to bargain, with actual selling prices 10% to (if you're an outstanding bargainer on a slow day) as much as 30% off.  Most vendors speak several languages and those who don't are very forgiving of broken Italian.  There are even a few places to sit and eat while you compare bargains or watch the crowds pass by.

The flea market occupies both sides of the street for MANY blocks parallel to the Tiber. It could take literally hours to see everything, and while there is a lot of junk, the serious shopper will go home with one or two real treasures.  Bartering and patience is a must.

When you get tired, walk 2 blocks away from the river and the Tram (trolley) nearest Porta Portese will take you to downtown Rome. Also, take an opportunity to check out a local trattoria in the beautiful Trastevere neighborhood. 

Watch for pickpockets.  There are not any obvious ones early in the season, but the crowds and many places to vanish into between shops makes it a place to use caution with your wallet or purse.