Updated December 9, 2013
Update December 2014: this year's market in Piazza Navona will run in a much reduced form. Only a reduced number of stalls (approximately 10) will populate piazza Navona (rather than the usual 100+), because there is a protest by the lobby of the stall owners, against Rome's administration. There will be carousels and artists anyway, so still worth going if you're in Rome.
In your calendar of the street markets and shopping low cost in the Eternal City, don't forget to pinpoint a visit to the beautiful Christmas market in Piazza Navona. What dates? This year it's going to start December 1, 2014 and it will be there until January 6, 2015, day of the epiphany. The market is open every day from 9.00 to 21.00.
The piazza with Bernini's fountain of the Four Rivers, already wonderful to visit in any period of the year, will sparkle with all kinds of Christmas decorations, gifts, food delicatessens, toys for kids, live music and carousels. It is very popular, and very crowded around the topic dates (right before Christmas and right before the Epiphany). I went last year few days before Christmas, there were so many people that I was walking at snail speed!
This Christmas market will fill Piazza Navona with very big stalls (much higher, bigger and more stuffed than the usual stalls I saw in other street markets) with products that will vary slightly according with the period:
In the belt surrounding the piazza, in Via dei Coronari, Via dell'Orso and Sant'Eustachio, you'll have the chance to taste and buy many traditional italian and roman foods for the holidays (like torrone, panettore, pandoro, panpepato, caldarroste) and live the magic of Christmas in the historic center of Rome, between many concerts, gospels and artists.