Every weekend, locals head to the Kolo Bazar in Warsaw's Wola district. Perhaps this flea market is more frequented by tourists in warmer months, but as a visitor in January, I felt like I had discovered a real hidden gem.
It sells mostly furniture on Saturday mornings and everything else one expects at a flea market on Sunday mornings. We went on a Sunday and still found a good amount of furniture along with vintage furs, books, old postcards, clocks, china, jewelry, silverware, and much more.
It's recommended to haggle for purchases within the market, although the sellers right outside might give you a fairer price. I bought two old photographs from the 1920s in the market for five zloty (about $1.30) without haggling.
Scattered among the stalls are some food stands and several little restaurants and bars. You can also use the clean public restroom for 2 zloty. Although the market is in an outer district, it's easily accessible by several tram lines: number 24 going to and from Warsaw Centrum and number 23 going to and from Old Town.