This famous flea market only happens twice a year, so if you happen to be free on December 15-16 or January 15-16, be sure to head over (9am - 8pm).
The Boro-Ichi, translated as Junk Market, is a big two-day event that attracts a lot of people, as they turn the streets into a massive pedestrian zone of vendors selling everything from sake to second-hand kimonos. It has also been running for over 400 years, and is considered a great cultural event in the Setagaya area.
It starts just outside of Setagaya Station on the Setagaya Line, where tents have been set up to allow local vendors to sell homemade pickles, dried fruits and vegetables, and a variety of snacks. Further up the road there are hundreds of stalls selling second-hand items.
Flea markets are very popular in Japan, as people here shop compulsively and second-hand goods are usually very good quality; not the stained old t-shirts you tend to find in the Salvation Army in the US. At the Boro-Ichi you can pick up some high quality suits, shoes, leather jackets, and bags.
Traditionally Japanese people clean house at the end of the year, and throw away, recycle, or sell things they no longer need. You can find some really beautiful and traditional Japanese items, such as laquer ware, kitchen goods, tea sets, sake bottles and cups, second-hand kimonos for very very cheap, decorations, art, and so on. From decorative swords to paper umbrellas, hand-made dog sweaters and jewelry, you can spend a good few hours ambling around from stall to stall, looking at all the weird and quirky items for sale.
A number of vendors also set up shop along the streets, serving drinks and typical street food such as yakisoba, okonomiyaki, dango and takoyaki, so even if you don't buy anything, it very nice for tabe-aruki (たべあるき）ー walking and eating!